Methicillin-resistant Staphylococcus aureus: clinical manifestations and antimicrobial therapy

Authors

  • B. A. Cunha

    1. Infectious Disease Division, Winthrop-University Hospital, Mineola, New York and State University of New York, School of Medicine, Stony Brook, New York, USA
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Corresponding author and reprint requests: B. A. Cunha, Infectious Disease Division, Winthrop-University Hospital, Mineola, New York and State University of New York, School of Medicine, Stony Brook, New York, USA
Tel: +(516) 663 2505
Fax: +(516) 663 2753

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is a common skin coloniser and less commonly causes infection. MRSA colonisation should be contained by infection control measures and not treated. MRSA infections cause the same spectrum of infection as MSSA infections, i.e., skin/soft tissue infections, bone/joint infections, central IV line infections, and acute bacterial endocarditis (native valve/prosthetic valve). There is a discrepancy between in-vitro sensitivity and in-vivo effectiveness with MRSA. To treat MRSA infections, clinicians should select an MRSA drug with proven in-vivo effectiveness, i.e., daptomycin. Linezolid, quinupristin/dalfopristin, minocycline, or vancomycin, and not rely on in-vitro susceptibility data. For MRSA, doxycycline cannot be substituted for minocycline. Linezolid and minocycline are available for oral administration and both are also effective in treating MRSA CNS infections. Vancomycin is being used less due to side effects, (increasing MICs/resistance, VISA/VRSA), and increased VRE prevalence. The most potent anti-MRSA drug at the present time is daptomycin. Daptomycin is useful when rapid/effective therapy of MRSA bacteraemia/endocarditis is necessary. Daptomycin is also useful to treat persistent MRSA bacteraemias/MRSA treatment failures with other drugs, i.e., vancomycin. There is no difference in virulence between MSSA and MRSA infections if treatment is started early and with an agent that has in-vivo effectiveness.

Introduction

Since the late 1970s, methicillin-resistant Staphylococcus aureus (MRSA) has been reported as the cause of outbreaks in hospitals throughout the world. MRSA was first reported in 1961. Initially, only outbreaks were recognised, as screening for methicillin resistance was not routinely performed in hospital laboratories. It was gradually appreciated that MRSA, like methicillin-sensitive Staphylococcus aureus (MSSA), could colonise or infect patients. MRSA strains were not found to be more virulent than MSSA strains and caused the same spectrum of infection. MRSA, like MSSA, primarily colonises the nares and the skin of individuals. Initially, MRSA infections were encountered in teaching hospitals, but gradually MRSA colonisation and infection also spread to non-teaching hospitals. Currently, MRSA colonisation/infection is encountered in small community hospitals, chronic care facilities and even within the community. Patients colonised in hospitals, when discharged, can spread strains throughout the community and thus colonise or infect non-hospitalised patients. Conversely, patients who have been colonised or infected in the community can introduce MRSA into a hospital when they are admitted. Many outbreaks have involved inter-hospital spread from colonised patients or medical staff to other patients, resulting in either episodic outbreaks or variable levels of MRSA colonisation/infection in institutions. Patients who are colonised or infected represent the MRSA reservoir in hospitals and, to a lesser extent, in the community [1–3].

Epidemiology

Colonisation

MRSA colonises the nares and the skin and, to a lesser extent, the urine or faeces. Hand carriage of MRSA by medical personnel is important in patient to patient spread. MRSA colonisation of respiratory secretions in ventilated patients and in burn units may contribute to airborne dissemination via droplets between medical staff and colonised/infected patients. MRSA colonisation is difficult to eradicate compared with the treatment of MRSA infections. Difficulty in the eradication of the carrier state of MRSA relates to the relatively few antibiotics active against MRSA that are able to penetrate nasal secretions in sufficient concentration to eliminate nares colonisation. As a general principle of infectious disease, colonised patients should not be treated; treatment should be limited to those who are infected.

The use of some antibiotics is associated with an increase in MRSA colonisation, e.g., ceftazidime. An increased prevalence of MRSA is evident in institutions that rely heavily on the use of certain antibiotics. The containment of MRSA-colonised patients primarily depends on the adherence to proven infection control measures. The same methods used to control the spread of MSSA are useful in controlling the spread of MRSA [2,4–9].

Infection control methods

At the present time, MRSA is an endemic nosocomial pathogen in many hospitals worldwide. Patients suspected of being colonised with MRSA should have their nares, as well as other appropriate body sites, cultured for MRSA. The critical infection control containment measure for MRSA is early recognition of the colonised patient who presents a greater threat of spreading MRSA in the hospital than the infected patient. Infected patients are easily recognised as infected and are handled with the appropriate precautions. The colonised patient has no signs or symptoms of infection, leading to relaxed infection control measures, and, in turn, patient to patient or patient to healthcare worker transmission within the institution [10–14].

Identification and detection of mrsa

In the early 1960s, when methicillin was introduced as an anti-staphylococcal antibiotic, the first isolates of MRSA were reported. It was quickly noted that, in cultures of clinical specimens of MRSA, there were populations of staphylococci with varying sensitivity to methicillin. Most isolates in such specimens were sensitive to methicillin, but a minority of these MRSA strains were highly resistant. This phenomenon is termed heterogeneous MRSA resistance. Conversely, homogeneous resistance indicates uniform resistance to methicillin. Methicillin was quickly replaced by less toxic anti-staphylococcal penicillins, e.g., oxacillin. For this reason, oxacillin became the antibiotic used to detect methicillin resistance. Staphylococcal strains reported as oxacillin resistant are methicillin resistant [2].

In MRSA susceptibility testing, temperature has a profound effect. Concerning heterogeneous MRSA resistance, most isolates that grow at 37°C appear to be sensitive to methicillin. However, if the incubation temperature is decreased to 25°C or 30°C, the number of MRSA isolates will increase.

If incubation is carried out at high temperature, i.e., 40°C, the majority of heterogeneous MRSA resistance isolates appear to be sensitive. For this reason, the incubation of cultures for antibiotic susceptibility testing should be performed at 35°C, which is optimal for the susceptibility testing of S. aureus. Heterogeneous MRSA resistance is also a time-dependent phenomenon. Prolonged incubation, i.e., 48 h, is recommended for the optimal detection of heterogeneous MRSA. Although 48 h is optimal, 24 h is acceptable and a 24-h incubation period is used routinely in medical clinical laboratories.

Inoculum size is also important in MRSA testing. A small inoculum may contain mixed populations of MSSA strains, whereas a larger inoculum is necessary to detect some isolates with methicillin resistance. Heteroresistant MRSA detection is also optimised with increased osmolarity of the medium. For this reason, the addition of sodium chloride (5%) or ammonium sulphate (7%) optimises MRSA detection.

In-vitro susceptibility testing is also affected by pH. The detection of methicillin resistance is suppressed at pH 5.2; therefore, susceptibility testing for MRSA should be carried out at a higher pH.

Disc diffusion is the most widely used method of MRSA susceptibility testing; alternatively, Mueller–Hinton agar may also be used. Plates are incubated for 24 h at 35°C for best results. Inhibitory zones should be carefully examined for isolated colonies or minimal growth. Intermediate methicillin resistance is a rare phenomenon amongst isolates. Intermediately resistant strains usually result from the hyperproduction of β-lactamase and should be considered as resistant for clinical purposes. The broth microdilution technique is also used by laboratories, but this method is less sensitive for the detection of heteroresistant MRSA isolates.

Incubation temperatures of 30–35°C and the extension of incubation for 48 h optimise the detection of MRSA strains. Usually, sodium chloride is added to the medium to optimise the detection of methicillin resistance. Five per cent sodium chloride is the optimal concentration to detect methicillin resistance and is better than either 2% or 10% sodium chloride. The inoculum used should be large and the broth should be adjusted to a McFarland 0.5 standard of turbidity. Either oxacillin or nafcillin may be used instead of methicillin for testing, but other anti-staphylococcal antibiotics, i.e., dicloxacillin or cloxacillin, should not be used. When using oxacillin for MIC testing, susceptible strains have a MIC of ≤ 2 µg/mL, and those with a MIC of ≥ 4 µg/mL should be considered as resistant.

Many microbiology laboratories use automated antimicrobial susceptibility testing systems. The accuracy of such systems in detecting methicillin resistance depends on the methodology employed in the system. The more closely the methodology resembles that used in standard non-automated testing, the more likely the results are to be valid. Any MSSA isolates with unusual susceptibility patterns, identified with an automated system of susceptibility testing, should be re-tested using standard methods.

The usual mode of methicillin resistance, i.e., resistance to oxacillin, is due to the acquisition of the mecA gene that codes for PBP 2a, which has a low affinity for β-lactam antibiotics. Alternatively, hyper-β-lactamase-producing strains or modification of existing penicillin-binding proteins may also result in methicillin resistance. In the clinical laboratory, mecA-positive strains are not distinguished from other isolates and are all reported as MRSA. Some investigators have successfully detected all types of oxacillin resistance using a screening procedure, employing cefoxitin or moxalactam, testing by disc diffusion. Oxacillin resistance is present if the zone of clearing around a cefoxitin disc is 27 mm, or is < 24 mm surrounding a moxalactam disc. In the near future, the molecular detection of the mecA gene will become the standard for identifying MRSA. Currently, this involves a research laboratory procedure, but hopefully it will become available commercially in the near future [2,15,16].

Clinical presentation

Overview

During the 1970s, when MRSA infections were being encountered with increasing frequency, it was quickly understood that the spectrum of MRSA paralleled that of MSSA. The other important lesson from the initial worldwide MRSA outbreaks in the 1970s was therapeutic rather than diagnostic. Although the clinical spectrum of MSSA infections had long been recognised and effective treatment strategies had been developed, patients with MRSA did not respond to most antibiotics used at the time (β-lactams) which, although reported as being effective in vitro, were ineffective in vivo. However, from the clinical perspective, the most common clinical infections caused by MRSA included surgical wound infections, both primary and secondary bacteraemias, intra-abdominal/pelvic abscesses, osteomyelitis, prosthetic joint infections and, rarely, nosocomial pneumonias. Initially, it was thought that, because patients with MRSA infections did not respond to antibiotics, MRSA infection was somehow different from its MSSA counterpart. This was later shown to be a result of differences in the effectiveness of therapeutic agents rather than a different spectrum of infection or a more virulent strain of S. aureus[2,17–20].

Sites commonly colonised by MRSA but uncommonly infected

MRSA, being part of the skin flora, may colonise any mucosal surface contiguous with MRSA-colonised skin. Accordingly, MRSA may colonise wounds and burns, as well as respiratory secretions, urine and faeces. MRSA infections involving the genitourinary system, excluding haematogenously derived MRSA abscesses, are distinctly unusual. S. aureus is not a normal part of the faecal flora in patients who have not been treated with antibiotics or who have not been previously hospitalised. If such patients become colonised, they do not develop manifestations of gastrointestinal infection. S. aureus has long been known to be a cause of lung abscesses. Staphylococcal lung abscesses may occur from staphylococcal bacteraemia or from a contiguous source or staphylococcal chest wall infection, resulting in staphylococcal empyema. S. aureus, both MSSA and MRSA, frequently colonises the respiratory secretions of intubated patients on ventilators. Colonisation of respiratory secretions is common after prolonged hospitalisation and broad-spectrum antimicrobial therapy, which eliminates much of the normal flora, permitting MRSA to become the predominant colonising organism in respiratory secretions. S. aureus hospital-acquired pneumonia is rare, compared with MRSA colonisation of respiratory secretions, which is a daily occurrence. Staphylococcal pneumonia has long been recognised as a complication of viral influenza. Either MSSA or MRSA may be associated with staphylococcal pneumonia in this context [1–3,19](Table 1).

Table 1.  Spectrum of MRSA infection
COMMONUNCOMMON
• Skin/soft tissue infections• Skin/soft tissue abscesses
• Bone/joint infections• Intra-abdominal abscesses
• Central IV-line infections• Perinephric/intra-nephric abscesses
• Native valve endocarditis• Pelvic abscesses
• Prosthetic valve endocarditis• Lung abscesses
• CNS shunt infections• Ventilator-associated pneumonia (rare)
• Meningitis/brain abscess 

Sites commonly infected/colonised

Skin and soft tissue infections

Skin and soft tissue infections are clearly the most common clinical MRSA infection. Infection may be limited to the upper layers of the dermis, e.g., cellulitis, or may involve deeper structures, e.g., soft tissue abscesses. Staphylococcal skin infections may be differentiated clinically from Group A streptococcal skin infection by the presence of bullae and the absence of systemic symptoms. High fever, chills and lymphangitis indicate a Group A streptococcal aetiology. Staphylococcal abscesses of the soft tissues occur following blunt or open trauma and may be due to MSSA or MRSA. Staphylococcal soft tissue abscesses are common in patients with diabetes mellitus and may be due to MSSA or MRSA. Staphylococcal abscesses may be accompanied by high fever and chills. With staphylococcal cellulitis, chills are not prominent, in contrast with streptococcal cellulitis. MRSA may also present as toxic shock syndrome in patients colonised or infected with toxigenic strains of MRSA [1,18].

Bone and joint infections

S. aureus is the most common pathogen in bone and joint infections. Septic arthritis and acute osteomyelitis are most frequently due to S. aureus. Bone and joint infections may be due to either MSSA or MRSA, depending on local geographic patterns. The clinical presentation of MRSA septic arthritis or osteomyelitis is indistinguishable from similar infections caused by other organisms.

Prosthetic joint infections, usually involving the hip or knee, are most often caused by strains of coagulase-negative staphylococci. However, S. aureus is second only to the coagulase-negative staphylococci as an important pathogen in prosthetic joint infections. MRSA may affect knee or hip prostheses. In contrast with low virulence organisms, e.g., the coagulase-negative staphylococci, S. aureus is more likely to present as acute prosthetic joint infection [2,19,21].

Endovascular infections

Staphylococcal endocarditis usually presents as acute bacterial endocarditis (ABE). The increase in MRSA ABE usually parallels the general increase in MRSA in the geographical area of patients. Staphylococcal ABE may occur on normal valves and result from a staphylococcal bacteraemia. Nosocomial ABE may occur in patients recently subjected to intracardiac monitoring devices. Staphylococcal endocarditis has also been related to intravascular catheter or pacemaker wires and may involve normal or prosthetic heart valves. Staphylococcal ABE occurs primarily in intravenous drug abusers (IVDAs). Septic emboli are common in these patients and there is a predominance of right-sided valvular involvement. The prognosis is better for IVDAs than for normal hosts with a similar extent of intracardiac infection [2,18].

Intravenous line infections are directly related to medical progress, as lines have become essential in providing access and for monitoring patients. However, these devices bypass normal defence mechanisms, e.g., the skin. The skin is often colonised with staphylococci, which may gain access to the catheter and bloodstream via the catheter entry wound. Staphylococcal bacteraemias may be self-limiting or result in localised or metastatic infection. Some patients will develop overwhelming septicaemia with a fatal outcome. The clinical expression of staphylococcal bacteraemia depends on the entry point of the staphylococcus, the inoculum size, the underlying condition of the host and host defence factors. Patients with staphylococcal bacteraemia or endocarditis usually have a demonstrable focus of infection involving the skin/soft tissues or emanating from a bone/joint infection. In the absence of a recognisable focus for staphylococcal bacteraemia, an endovascular infection, e.g., endocarditis, should be considered. In a minority of patients with staphylococcal bacteraemias, no focus can be demonstrated. Because staphylococcal bacteraemias not infrequently eventuate in ABE, early treatment is essential. If intravenous devices are thought to be the cause of the bacteraemia, they should be removed or replaced [7,19].

Vertebral osteomyelitis

Vertebral osteomyelitis is a complication of S. aureus bacteraemia. Staphylococci in the bloodstream will lodge in previously damaged joints or bones. A staphylococcal infection/abscess often occurs in a vertebral body/disc previously damaged by arthritis. Elderly patients may not infrequently develop a paravertebral or epidural abscess following staphylococcal bacteraemia as the initial manifestation of vertebral osteomyelitis [2,18,22].

Therapy of mrsa infections

General concepts

The therapeutic approach to patients with MRSA infection depends on the site of the infection and the choice of antimicrobial agent with in-vivo activity against MRSA. As with other infectious diseases, the severity of the infection has important prognostic implications, but is not an important factor in antibiotic selection. The site of infection, and the removal of an infected device or drainage of an abscess, are more important than antimicrobial therapy, as in the case of other staphylococcal infections [2](Table 2).

Table 2.  MRSA facts and fallacies
FallaciesFacts
  1. IV = intravenous; PO = oral; PK = pharmacokinetic; PD = pharmacodynamic

• MRSA colonization is not important.• MRSA colonization is the primary mode of spread from patient to patient, and from patient to staff, and staff to patient.
• MRSA colonization should be treated.• MRSA colonization is difficult to eliminate, and should not usually be treated.
• Antibiotics reported as sensitive to MRSA in vitro may be used for therapy.• Only daptomycin, linezolid, quinupristin/ dalfopristin, minocycline, and vancomycin, are consistently effective against MRSA in vivo.
• MRSA strains are more virulent than MSSA strains.• MRSA causes the spectrum of infection as MSSA, and are not more virulent if treated promptly/properly.
• IV therapy for MRSA is more effective than oral therapy.• IV and PO therapy are equally efficacious if the oral agent has high degree of bioavailability and the same PK/PD as its IV counterpart.

Skin/soft tissue infections and bone/joint infections

The non-medical treatment of skin/soft tissue infections and bone/joint infections due to MRSA is the same as that for MSSA infections. The surgical drainage of an abscess is as important as appropriate antimicrobial therapy. The drainage of septic joints is important to minimise damage to the synovium and articulating surfaces. Direct instillation of antibiotic into the joint is unnecessary with agents that penetrate well into synovial fluid. The treatment of acute osteomyelitis, usually due to S. aureus, is entirely medical with appropriate anti-staphylococcal therapy. Chronic osteomyelitis may be due to S. aureus, but is more frequently due to aerobic Gram-negative bacilli. However, if it is determined that chronic osteomyelitis is staphylococcal, appropriate surgical debridement should be carried out. Without adequate surgical debridement, cure of chronic osteomyelitis is not possible with antimicrobial therapy alone [2,20].

Central intravenous line infections

The treatment of intravenous line infection due to MRSA first depends on the establishment of a diagnosis. MRSA line infections present with otherwise unexplained fevers, and a catheter entry site may or may not appear to be infected. If the catheter insertion site looks infected, the central intravenous catheter should be removed as soon as possible. If no other source of fever can be determined and the intravenous central line has been in place for an extended period of time, i.e., > 2 weeks, a presumptive diagnosis of intravenous line infection should be entertained. The diagnosis of intravenous line infection due to MRSA or other agents is confirmed by semi-quantitative catheter tip culture after removal; if the removed catheter tip grows ≥ 15 colonies using the Maki or Cleary semi-quantitative technique, and the patient has blood cultures drawn at the same time as the central line is removed, which are positive for the same organism, intravenous line infection is confirmed. If a central line tip is removed with possible intravenous line infection as the diagnosis, and the catheter tip is negative or grows ≤ 15 colonies, the criteria are not met. Similarly, a positive intravenous catheter tip culture with ≥ 15 colonies occurring in association with negative blood cultures is not diagnostic of intravenous line infection. The likelihood of central line infection is a function of time, assuming that an aseptic technique has been used during catheter insertion. After the diagnosis has been confirmed, the main therapeutic intervention is removal of the central catheter. Because intravenous line infections due to S. aureus, either MSSA or MRSA, have the potential to cause endocarditis, antimicrobial therapy is given for 2–4 weeks following central line removal. The duration of therapy for MSSA and MRSA line infections has not been determined [19,23,24].

Acute bacterial endocarditis

Staphylococcal endocarditis may occur as a result of a staphylococcal bacteraemia seeding a normal valve, e.g., the aortic valve, or as a result of damage to the endothelium or heart valves from an intravascular device positioned in the right heart. The other group of patients with an increased incidence of staphylococcal endocarditis are IVDAs. The clinical presentation of staphylococcal ABE in IVDAs is milder and subacute in contrast with the fulminant course of ABE in normal hosts. The effective treatment of staphylococcal ABE depends on an accurate diagnosis, i.e., positive blood cultures plus a cardiac echocardiogram that demonstrates one or more vegetations or intramyocardial abscesses. A myocardial abscess, depending on its position, may present as fever of unknown origin, valvular dysfunction or various degrees of heart block. If the staphylococcal ABE is due to an intravascular device in the right heart, it should be removed. Myocardial abscesses should be drained if they are large and surgically accessible. Cardiac valves, which are incompetent, should also be replaced surgically with prosthetic valves. The antimicrobial therapy for staphylococcal endocarditis differs according to the setting. IVDAs with staphylococcal ABE have been managed using oral anti-staphylococcal therapy with excellent results. Although the initial treatment of endocarditis in non-IVDAs has traditionally been intravenous, there is a trend towards treatment of patients with oral antibiotics that have favourable pharmacokinetics, making oral treatment as effective as intravenous therapy at the present time, daptomycin is arguably the most effective drug for MRSA ABE/PVE [2,18].

Staphylococcal meningitis

Staphylococcal meningitis may occur as a complication of staphylococcal ABE as the result of meningeal seeding. Staphylococcal brain abscesses may occur following open or closed neurosurgical trauma. Staphylococcal shunt infections may complicate neurosurgical procedures. Small staphylococcal abscesses of the brain may be managed medically, but large abscesses should be drained if neurosurgically possible. Infected central nervous system (CNS) shunts should be removed and, if due to MSSA or MRSA, anti-MRSA therapy that penetrates the cerebrospinal fluid in therapeutic concentrations should follow, i.e., linezolid, minocycline or high dose intrathecal vancomycin (30 mg/kg/day) [2,18,19].

Anti-mrsa antimicrobial agents

The therapy of MRSA infections is complicated by the fact that relatively few agents with proven in-vivo efficacy against MRSA are available. While many antibiotics appear to have anti-MRSA activity in vitro, they are ineffective in vivo or only sporadically effective. Therefore, clinicians should not rely on in-vitro susceptibility testing to determine antibiotic selection for anti-MRSA therapy. There are only four drugs that have demonstrated consistent activity against MRSA in vivo. Four antimicrobial agents with a consistent and high degree of anti-MRSA activity are quinupristin/dalfopristin, minocycline, daptomycin, linezolid and vancomycin. The anti-MRSA effects of trimethoprim–sulfamethoxazole (TMP-SMX) are variable. Rifampin is a potent anti-staphylococcal antibiotic, but its activity in MRSA infections remains to be demonstrated [21,22,24–30].

Given the four agents available to treat MRSA infections, the clinician must choose on the basis of the assessment of several factors. Antibiotic selection for MRSA infection or any other infection depends on an assessment of the activity of the agent, its pharmacokinetics, its safety profile, its resistance potential and its cost to the patient or institution.

The availability of the antibiotic as an oral formulation as well as an intravenous preparation is also important. Oral therapy is usually less expensive than its intravenous counterpart, and provides the clinician and patient with greater flexibility in an effective therapeutic regimen. Daptomycin, vancomycin, and quinupristin/dalfopristin are only available intravenously for the treatment of MRSA infections. Minocycline and linezolid are available in both oral and intravenous formulations. The use of quinupristin/dalfopristin is occasionally complicated by painful myalgias. Vancomycin has the advantage of a long therapeutic experience, but has disadvantages, i.e., limited penetration into bone and CSF (∼15% of simultaneous serum levels), and a variety of side-effects. Minocycline has the advantage of relatively low cost, and has the ability to penetrate the CNS. Minocycline has been used primarily to treat MRSA endocarditis in IVDAs and staphylococcal CNS infections.

Linezolid is available intravenously and orally, which is an important advantage. Orally administered linezolid has the same pharmacokinetic profile, resulting in the same serum/tissue concentrations, as the same dose given intravenously. Linezolid also has the advantage of excellent CNS penetration. Linezolid has been used successfully to treat MRSA ABE, CNS infections and a wide variety of MRSA infections not involving the heart or CNS, e.g., MRSA hospital-acquired pneumonia. One advantage of linezolid is its relatively low cost when given orally, compared with intravenously administered vancomycin [2,18,30] (Table 3).

Table 3.  Clinical comparison of anti-MRSA antibiotics
Anti-MRSA AgentUsual Adult Dose*Side EffectsAdvantagesDisadvantages
  1. *Normal renal/hepatic function

  2. Adapted from: Cunha BA. Antibiotic Essentials. Royal Oak, MI, 2005.

Daptomycin4 mg/kg (IV) q24 h (skin/soft tissue) 6 mg/kg (IV) q24 h (bacteraemia/endocarditis)NoneMost effective/rapidly bactericidal MRSA antibiotic
Resolves MSSA/MRSA bacteremias faster than vancomycin/other MRSA drugs
No ↑ VRE
q24 h dosing CrCl >30; q48 h dosing CrCl ≤30 Little/no resistance potential
No PO form
Linezolid600 mg(IV/PO)ql2 hTransient/reversibleAvailable PO and IVRelatively expensive IV
Quinupristin/ dalfopristin7.5 mg/kg (IV) q8 hThrombocytopenia Painful myalgiasProven effectivenessNo PO form Painful myalgias Q8h dosing
Minocycline100–200 mg(IV/PO)q12 hSkin discoloration with prolonged useEffective in ABE in IVDAs Available PO and IV
Excellent CNS penetration Inexpensive
Limited experience
Vancomycinlg(IV)ql2 hNeutropenia Thrombocytopenia ‘‘Red Man’’ syndromeLong clinical experience↑ MICs/resistance (VISA/VRSA)
No PO form for MRSA
↑ VRE
IV vancomycin more expensive than PO linezolid

Conclusion

MRSA has emerged worldwide as an important nosocomial pathogen since the 1970s. Over the years, laboratory methodology to detect methicillin resistance has been widely introduced and standardised, permitting accurate detection of MRSA strains from clinical specimens. Epidemiologically, MRSA occupies the same ecological niche in hospitals and the community as does MSSA. As S. aureus is a normal denizen of the nares/skin, it is not surprising that most infections due to MRSA are introduced into the skin, soft tissues or bloodstream from a skin source. It has also become apparent that, as with MSSA, the majority of MRSA isolates represent colonisation rather than infection. Because colonisation is the usual mode of MRSA expression in the clinical context, the infection control implications are important. TMP-SMX and mupirocin nasal ointment have been used with variable success, but infection control methods remain the preferred way to minimise MRSA transmission within an institution. Another factor influencing the prevalence of MRSA in an institution is the selection of antibiotics on the hospital formulary. Certain antibiotics are known to increase the prevalence of MRSA, e.g., ciprofloxacin, ceftazidime, imipenem. Institutions with high rates of prevalence of MRSA should consider restricting or reducing the usage of these agents after other methods have been tried.

The relative proportion of MSSA vs. MRSA causing colonisation/infection varies by geographical area. The empirical treatment of S. aureus infections depends on local epidemiological patterns. If the majority of S. aureus strains in a community are MSSA, initial empirical therapy should be with an agent which has a high degree of activity against MSSA. Conversely, if the majority of S. aureus isolates in a community are of the MRSA variety, initial empirical therapy should be with one of the agents known to be effective against MRSA in vivo[2,30].

With MRSA, unlike MSSA, there is a discrepancy between the in-vitro susceptibility and in-vivo effectiveness. Clinicians should be aware that they should not rely on susceptibility testing to select an antibiotic to treat MRSA infections. Treatment options are based on the antibiotic activity, pharmacokinetics, resistance potential, side-effect profile and cost. The preferred MRSA drugs are daptomycin, linezolid or quinupristin/dalfopristin. Linezolid or minocycline may be initially prescribed orally, or prescribed orally to complete an intravenous regimen. The primary use of minocycline in the treatment of MRSA is for ABE and IVDAs, for CNS infections and for those patients unable to tolerate vancomycin high dose (30 mg/kg/day) or linezolid.

Daptomycin is replacing vancomycin as the preferred drug for serious/systemic MRSA infections. The prevalence of vancomycin-resistant enterococci increases as a result of vancomycin therapy, and this effect is most pronounced with intravenous, as opposed to orally administered, vancomycin. The other disadvantage of vancomycin is that it cannot be given orally for serious/systemic MRSA infections. Oral vancomycin use is limited to the treatment of Clostridium difficile diarrhoea. Although vancomycin may be used to treat CNS staphylococcal infections, linezolid, administered both orally and intravenously, shows better CNS penetrance. Daptomycin is effective against MSSA and MRSA. Daptomycin is bacterial and appears to clear MRSA bacteremias more rapidly than other anti-MRSA antibiotics. The daptomycin dose is 4 mg/kg/day (IV) for skin/soft tissue infections, and 6 mg/kg/day (IV) for S. aureus bacteremias or endocarditis. The dose of daptomycin is the same in mild/moderate renal insufficiency (CrCl > 30 ml/min), but either the 4- or 6-mg/kg (IV) dose should be given every 48 h if the CrCl < 30 ml/min. The more serious/life threatening the MRSA infection, the more clinicians should select daptomycin. Daptomycin is also useful in MSSA/MRSA unresponsive to other agents (Figure 1).

Figure 1.

MRSA PVE complicated by aortic paravalvular abscess [36].

Surgical drainage or device removal is as important as antimicrobial therapy in the treatment of MRSA infections, and remains the cornerstone of non-antibiotic therapy [2,30,32–37].

Ancillary