An intubated 7‐month‐old infant with a retropharyngeal abscess and multidrug‐resistant Streptococcus mitis

Abstract The profile of antimicrobial resistance (ie, antibiogram) may be disparate between children and adults. An infant developed severe deep neck infection with a multidrug‐resistant microbe. The microbe was more drug‐resistant in children than in adults, in our hospital. Treatment of a child should be guided by the antibiogram obtained from children.

an alternative differential diagnosis was deep neck infection. From day 1, we started intravenous cefotaxime at 150 mg/kg/ day.
On day 2, however, her respiratory distress worsened, with severe retractions and respiratory acidosis. Enhanced computerized tomography (CT) revealed a right-sided peritonsillar abscess and retropharyngeal abscesses ( Figure 1). She was intubated immediately. An incision of the posterior pharyngeal wall yielded copious pus.
Empirically, we replaced cefotaxime with 10 mg/kg/day of meropenem from day 3. Additionally, on day 5, 300 mg/ kg/day of ampicillin was started. A bacterial culture of the pus from the retropharyngeal abscess, which was incised on day 2, identified Streptococcus mitis on day 6. We replaced ampicillin with 40 mg/kg/day of clindamycin from day 8. Subsequently, a drug susceptibility test using an EIKEN FROZEN PLATE (Eiken Kagaku) reported resistance of the S mitis to multiple drugs ( Table 1). The patient was extubated on day 10. Culture of the blood sample, which was collected on day 1, did not yield any microbe. She was discharged on day 24, with 9 mg/kg/day of oral tebipenem prescribed for 14 days. She has not had any relapse or severe infection to date.

| DISCUSSION
Our patient initially showed only mild signs of an upper respiratory infection, but quickly deteriorated. Peritonsillar and retropharyngeal abscesses were diagnosed by enhanced CT. In our facility, 159 cases of peritonsillar abscess were treated between 2013 and 2017. During this period, only 11 cases of retropharyngeal abscess were treated here ( Table 2). The rarity of this diagnosis may be due partly to the fact that it occurs in the deep tissue and relies upon imaging studies, particularly enhanced CT, to detect. Airway management was invasive: 5 cases (45%) were either intubated or tracheotomized. Treatment was surgical in most of the cases: 9 cases (82%) were incised.
Our patient was also intubated and incised. S mitis, which was cultured from the abscess, was resistant to multiple antimicrobials. S mitis belongs to viridans streptococci species, which are commensal to the oral cavity. 6,7 Viridans streptococci are the most common cause of deep neck infections, followed by Staphylococcus species and anaerobes. [2][3][4][5] Viridans streptococci are assumed to be susceptible to penicillins and cephalosporins. 8 Among viridans streptococci, however, S mitis is known to be highly resistant to multiple antibiotic drugs. [9][10][11][12][13][14][15][16][17] Cephalosporin resistance is highly prevalent in S mitis in Japan. 18 In addition, S mitis possesses various molecular strategies to compete with other bacteria in the oropharynx, promoting its virulence in opportunistic infections. 7,19 Consistently, infections with S mitis are associated with high morbidity and mortality, particularly among immunocompromised patients. [20][21][22][23][24][25] In contrast, our patient was immunocompetent and had not taken antimicrobials prior to this episode. This led us to review the medical records from our facility to understand the microbes that cause retropharyngeal abscess, especially S mitis. Table 2 reveals that retropharyngeal abscesses frequently yielded anaerobes, including Prevotella, Peptostreptococcus, and Fusobacterium, in patients from our hospital. Viridans streptococci were identified in three patients, including the present case. Three patients were infected with more than one of these four microbial groups. In particular, one patient was infected with all four microbial groups. These microbes, other than the S mitis in the present case, were susceptible in vitro to cefmetazole or sulbactam/ampicillin (data not shown). As a result, treatment with these antibiotic drugs had been effective in all cases, except the case reported here.
In contrast, S mitis in our patient was highly resistant to cephalosporins, macrolides, and fluoroquinolones and was intermediately resistant to penicillins (Table 1). This resistance may explain why our initial treatment with cefotaxime was not effective. Instead, S mitis in the present case was susceptible to meropenem and clindamycin. Therefore, our empirical choice of these drugs was fortunately adequate. The clinical microbiology laboratory of our hospital regularly issues the antibiogram-the profile of resistance and susceptibility to antimicrobials, based upon the bacterial culture database (La-vietal MB ® , Sysmex) which was introduced into our hospital in 2012. Upon our request, the clinical microbiology laboratory extracted records, accumulated between 2013 and 2017, that contained "S mitis". We examined the case record from the electric medical record system, corresponding to these bacteriological records. As is shown in Table 3, there was no significant difference in the origin of samples between pediatric and adult patients (P = .4080 by two-sided Fisher's exact test). The antibiogram for S mitis, stratified by age-group (ie, pediatric patients of 15 years or younger, or adult patients), was compiled. This age-stratified antibiogram showed that S mitis in our facility was more drug-resistant in the children than in the adults ( Figure 2). This may give a clue to our initial question: why retropharygeal abscess, a very rare disease in children, occurred in a healthy infant? The high prevalence of antimicrobial resistance of S mitis in the children in our community may have increased the probability that a healthy child encounters a multidrug-resistant strain of this species which caused a severer disease than susceptible strains. 26 It was previously suggested that frequent use of antibiotic treatment in pediatric and immno-compromised populations have increased the rate of drug-resistant viridans streptococci in these populations. 13,27  One of these isolates was from the spinal fluid. c One of these isolates was from nasal swab. Two (2), three (1), four (1), and one (7) Bacteria in abscess Prevotella (6), Viridans streptococcus (3), On average, a patient was treated with 1.6 antimicrobial drugs.
The utility of hospital-based antibiogram in assisting in the selection of antimicrobial drug has been well recognized. 28 In addition, it has been reported that the age greatly affects the profile of antimicrobial resistance. [29][30][31] Despite the fact that the use of hospital-based antibiogram is increasingly common, age-stratified antibiogram was rarely utilized. Agespecific information of antimicrobial resistance may provide useful input to the pediatric practice.

| CONCLUSION
We reported a pediatric case of deep neck infection caused by S mitis that was resistant to multiple antimicrobials.
Antibiogram stratified by age-group suggested that S mitis was more drug-resistant in children than in adults, in our hospital. Age-specific and locale-specific profile of antimicrobial resistance will be useful in guiding the treatment of children with severe infections.

ACKNOWLEDGMENTS
We are grateful to Takahisa Tabata for his advice. We thank to Makiko Matsumoto, Mana Tominaga, Naohisa Honjo, and Yuko Okazaki of the clinical microbiology laboratory of our hospital, for their assistance. We did not receive any specific funding.

F I G U R E 2 Antibiogram of
Streptococcus mitis in our hospital, stratified by the age-group. The percentage of resistant or intermediately resistant isolates, in the total number of isolates, is stratified by the age-group: children (15 years or younger) or adult. To the right of each bar, the denominator and the numerator represent the total number of isolates and the number of resistant/intermediately resistant isolates, respectively. Statistically significant difference (ie, P < .05) in the percentage is indicated with an asterisk. P-values were computed by two-sided Fisher's exact test