Bactericidal activity of moxifloxacin against pneumococci

Moxifloxacin is a new fluoroquinolone with a good activity against Gram-negative and Gram-positive bacteria. Against Streptococcus pneumoniae, moxifloxacin remains active on penicillin-resistant isolates [1,2].

In this work, we first tested the susceptibility of 150 isolates of pneumococci against moxifloxacin sampled from patients with otitis or sinusitis: 50 penicillin-susceptible isolates (minimal inhibitory concentrations [MIC] < 0.125 mg/L), 50 penicillin-intermediate isolates (MIC 0.125–1 mg/L) and 50 penicillin-resistant isolates (MIC > 1 mg/L). MICs were determined in Mueller-Hinton agar supplemented with 5% horse blood, according to CASFM guidelines [3]. Suspensions with a turbidity equal to a 0.5 MacFarland standard were prepared in Suspension Medium (Bio Mérieux, Mary-L’Etoile, France) and spotted in microplates. Suspensions were inoculated with a Steers replicator and incubated at 35 °C under CO₂ during 18 h. The lowest concentration of antibiotic inhibiting growth was considered as the MIC value. We then studied the bactericidal kinetics for five penicillin-susceptible, five penicillin-intermediate and five penicillin-resistant isolates, respectively. The bactericidal kinetics of the 15 isolates was studied by using concentrations based upon the MIC of moxifloxacin in the different isolates: 0.125 mg/L for 11 isolates, 0.25 mg/L for two, 0.06 mg/L and 2 mg/L for one. Isolates were grown in Mueller-Hinton broth and diluted in the same medium to obtain a final concentration of 10⁶−10⁷ of µ/mL. The concentrations used for moxifloxacin were adjusted at 1 and 4 times the MIC. A control without antibiotic was analyzed in the same conditions. A test for carry-over was performed in these conditions. Cultures were shaken, incubated at 35 °C under CO₂ and 100 µL were removed at 0 h, 1 h, 4 h, 7 h, 9 h, 24 h, plated on Columbia agar with 5% horse blood (Bio Mérieux) after dilution in NaCl 0.15 m and incubated at 35 °C under CO₂. Colony-forming units (CFU) were counted after serial dilutions. Bactericidal activity was defined as a reduction of 3 log₁₀ of CFU.

The MIC values are shown in Table 1. Each isolate of Strep. pneumoniae was susceptible to moxifloxacin, except one with a MIC at 2 mg/L. This isolate was defined as resistant to moxifloxacin according to a recent multicenter study which reported the MIC breakpoint for moxifloxacin of 1 mg/L [4]. The susceptibility of penicillin G did not influence the result of MIC 50 and 90. Moxifloxacin displayed a better activity than penicillin against penicillin-resistant isolates. In the majority of cases, moxifloxacin had lower MIC against penicillin-intermediate isolates.

Table 1.  MICs of moxifloxacin (mg/L) against Streptococcus pneumoniae

	Range MIC	MIC 50	MIC 90
Strep. pneumoniae Penicillin-susceptible
MIC < 0.125 mg/L	0.125–2	0.125	0.25
(50 isolates)
Strep. pneumoniae Penicillin-intermediate
MIC 0.125–1 mg/L	0.06–0.5	0.125	0.25
(50 isolates)
Strep. pneumoniae Penicillin-resistant
MIC > 1 mg/L	0.06–0.25	0.125	0.25
(50 isolates)

We found that the killing rate of moxifloxacin was faster at 4 times the MIC than at the MIC (Table 2). Although there were variations between isolates, penicillin-intermediate isolates were less susceptible to the bactericidal effect of the quinolone. Indeed, the decrease of CFU/mL was higher in penicillin-resistant isolates as compared to penicillin-susceptible and penicillin-intermediate isolates of Str. pneumoniae. At the MIC, there was occasionally no bactericidal activity after 24 h, whereas at 4 times the MIC, only one isolate was not killed after 9 h and none at 24 h. The time of 3 log₁₀ decrease of the moxifloxacin-resistant isolate was similar to the others. The bactericidal effect was very different between these isolates. Nevertheless, we can say that each isolate, except one, was killed at 4 times MIC after 9 h of incubation. As previously demonstrated in other studies, moxifloxacin shows a concentration-dependent killing rate against Strep. pneumoniae but also against other organisms that commonly cause respiratory, urinary, skin and soft-tissue infections [2,5–7]. Reductions in viable counts of 3 log₁₀ occurred in most cases, except three, within 24 h. These results indicate that moxifloxacin is active on wild-type isolates of penicillin-resistant isolates of Strep. pneumoniae.

Table 2.  Median bactericidal activity of moxifloxacin at 1 and 4 times MIC after 1, 4, 7, 9 h and 24 h of incubation. Results are expressed in decrease log₁₀ CFU/mL

	Moxifloxacin 1 x MIC	Moxifloxacin 4 x MIC
Strep. pneumoniae Penicillin-susceptible
1 h	0.25 (0.02–0.67)	0.51 (− 0.08–1.70)
4 h	1.37 (0.62–2.92)	2.39 (1.82–3.21)
7 h	2.11 (0.92–3.11)	4.10 (3.19–5.11)
9 h	2.87 (1.07–5.11)	4.66 (3.92–5.41)
24 h	4.11 (2.39–5.42)	5 (4.4–5.5)
Strep. pneumoniae Penicillin-intermediate
1 h	0.13 (− 0.19–0.92)	0.04 (− 0.11–0.22)
4 h	1 (0.84–1.37)	1.98 (1.35–2.52)
7 h	1.79 (1.18–2.31)	3.03 (2.08–4.09)
9 h	2.09 (1.28–2.52)	3.68 (2.65–4.86)
24 h	3.38 (2.56–4.93)	5.10 (4.93–5.15)
Strep. pneumoniae Penicillin-resistant
1 h	0.05 (− 0.05–0.16)	0.15 (− 0.03–0.65)
4 h	2 (0.61–2.60)	2.42 (1.86–3.21)
7 h	3.40 (1.83–4.84)	4.28 (3.13–5.14)
9 h	3.95 (2.43–5.14)	4.9 (3.65–5.41)
24 h	4.87 (3.43–5.5)	5.23 (4.73–5.5)