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We examined the relationship between penicillin susceptibility, peritoneal virulence in Swiss mice, and capsular type in a selection of 122 clinical Streptococcus pneumoniae isolates belonging to 24 serotypes. Regardless of the serotype, all 32 virulent strains were susceptible to penicillin, and all 41 strains with diminished susceptibility or resistance to penicillin were avirulent. The remaining 49 strains were both susceptible to penicillin and avirulent, irrespective of the serotype. On the basis of their capsular type and pathogenic behavior, strains fell into one of four groups. In the group consisting of serotypes 1, 3, and 4 (n = 16), strains were predominantly virulent (81.3%), and all were penicillin susceptible. In the serotype 6 group (n = 32), the frequency of virulence was significantly lower (34.4 versus 81.3%, P = 0.002), and strains were predominantly penicillin susceptible (71.9%). In the group composed of serotypes 9, 14, 19, and 23 (n = 50), all strains were avirulent, and 56% had decreased susceptibility (n = 12) or resistance to (n = 16) penicillin. The fourth group was heterogenous, as it pooled 24 strains of 15 different serotypes; in this group the frequency of virulence was 33.3%, and strains were predominantly penicillin susceptible (83.3%). These data point to a complex relationship between penicillin susceptibility and virulence in mice but do not entirely separate these characteristics from the role of the capsular type. The possibility that the mechanisms conferring penicillin resistance are related to those leading to a loss of virulence is supported by these findings.

We studied the relationship between in vitro bacteriological parameters [minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC) and killing rate, defined as the reduction in the inoculum within 1, 3 or 6 hr] and in vivo activity of amoxicillin against 12 strains of Streptococcus pneumoniae, with penicillin MICs of < 0.01 to 16 micrograms/ml, in a cyclophosphamide-induced neutropenic murine pneumonia model. Dose-response curves were determined for amoxicillin against each strain, and three quantitative parameters of in vivo amoxicillin activity were defined, i.e., maximal attainable antimicrobial effect attributable to the drug [i.e., reduction in log colony-forming units (CFU) per lung, compared with untreated controls], dose required to reach 50% of maximal effect and dose required to achieve a reduction of 1 log CFU/lung. We demonstrated a highly significant correlation between the dose required to reach 50% of maximal effect and MIC (Spearman r = 0.98, P < .0001) or MBC (Spearman r = 0.95, P < .0001) for amoxicillin against strains of S. pneumoniae with a wide range of amoxicillin MICs (0.01-8 micrograms/ml). Significant correlations between the dose required to achieve a reduction of 1 log CFU/lung and MIC (Spearman r = 0.98, P < .0001) or MBC (Spearman r = 0.95, P < .0001) were also observed. In contrast, there were no significant correlations between the maximal attainable antimicrobial effect attributable to the drug and MIC, MBC or killing rate or between killing rate and the dose required to reach 50% of maximal effect or the dose required to achieve a reduction of 1 log CFU/lung. We conclude that in vitro susceptibility test results (MICs and MBCs) correlated well with in vivo amoxicillin activity against pneumococcal strains, including highly penicillin-resistant strains, in this animal model. Furthermore, these data suggest that the estimated MIC breakpoints for amoxicillin against S. pneumoniae would be 2 micrograms/ml for intermediate-resistant and 4 micrograms/ml for resistant, although this remains to be confirmed in clinical studies.

In vivo synergy with beta-lactam antibiotics and aminoglycosides has been studied only with penicillin-susceptible Streptococcus pneumoniae strains. We evaluated the interaction between amoxicillin (AMX) and gentamicin (GEN) on the basis of in vitro checkerboard and time-kill curves and of findings in a mouse model of acute bacteremic pneumonia due to a highly penicillin-resistant and -tolerant S. pneumoniae strain of serotype 19 (penicillin and AMX MICs of 4 micrograms/ml; gentamicin MIC of 16 micrograms/ml). Checkerboard results at 18 h of incubation showed indifference. With regard to AMX alone, in vitro time-kill curves demonstrated synergy between AMX (1 microgram/ml) and GEN (16 micrograms/ml) at 5 and 8 h of incubation and for AMX (16 micrograms/ml) in combination with GEN (16 micrograms/ml) at 3, 5, and 8 h of incubation. In leukopenic mice, pulmonary killing curves after a single drug injection demonstrated that AMX (100 mg/kg of body weight) with GEN (16 mg/kg) was more effective than AMX alone (P = 10(-4). With repeated-dose treatment, a synergy was apparent at 8 h after four injections with AMX (100 mg/kg) in combination with GEN (8 or 16 mg/kg) (P < or = 0.05). The cumulative survival rate with AMX (100 mg/kg) every 8 h, combined with GEN (4 or 8 mg/kg) every 8, 12, or 24 h, was better than with AMX alone. Combined use of AMX and GEN may be a valuable therapeutic alternative for pneumococcal pneumonia due to highly penicillin-resistant S. pneumoniae strains.

Streptococcus pneumoniae and Haemophilus influenzae are the bacteria most frequently involved in bronchopulmonary and E.N.T. diseases. Experimental models are useful to analyse in vivo the physiopathological interactions between bacteria and lung tissue and to study the activity of various antibiotics in the focus of infection. Models for pneumococci are devised according to the virulence of the strain, the inoculation technique and the type of animal used. When strains of different virulence and hosts of different lineage are combined, several experimental models with different natural histories are obtained. These experimental models make it possible to study therapeutic measures at different stages, comparing the relative activities of fluoroquinolones, ampicillin and macrolides. For fluoroquinolones, no correlation has been found between in vivo and in vitro data in these models, but the effectiveness of new compounds has been studied. As regard Haemophilus influenzae, lung infection models are rare, either because their septicaemic component is too important or because they do not enable this organism to multiply. Moreover, the models obtained do not lend themselves easily to therapeutic trials and favour those antibiotics that have high serum concentrations. Developing a better model would enable us to improve our knowledge of the pathogenicity of this micro-organism.

We studied the pharmacokinetic parameters of four fluoroquinolones--ofloxacin, ciprofloxacin, temafloxacin and sparfloxacin--in a mouse model of Streptococcus pneumoniae-infected lung. After a single subcutaneous injection, bioactivities were determined concomitantly in non-infected and infected animals, at a time at which pneumonia was well developed. Fluoroquinolones exhibited generally good activity at the site of infection. Infection did not affect the tissue distribution of either drug. However, the differences observed in non-infected controls (greater AUCs and longer half-lives of temafloxacin and sparfloxacin in lung and serum) were accentuated in infected mice, with probable trapping of temafloxacin and sparfloxacin at the site of infection and more persistent activity in the lung. These data suggest that the anti-pneumococcal activity of temafloxacin, and to a lesser extent sparfloxacin, were greatly favoured by their pharmacokinetic behaviour in the infected lung.

We have compared the efficacy of temafloxacin against Streptococcus pneumoniae in experimental murine pneumonia models with that of ofloxacin and ciprofloxacin. Erythromycin and amoxycillin were used as reference agents. Two different strains of mice were used: Swiss mice develop acute pneumonia and die within three to four days, while C57B1/6 mice develop subacute pneumonia and die within eight to ten days. In both cases all animals quickly become bacteraemic. Mice were infected with approximately 10(5) cfu of a virulent S. pneumoniae strain, serotype 3 (P4241), by the intra-tracheal per oral route. In Swiss mice, subcutaneous treatments were initiated early (18 h post-infection) and given every 12 h for 72 h. A 79% cumulative survival rate was obtained in temafloxacin-treated mice (50 mg/kg); a figure not significantly different from those for erythromycin and amoxycillin but far higher than those for ofloxacin and ciprofloxacin (7% survivors). Pulmonary clearance of bacteria was consistent with the survival rates. Complete clearance of bacteria in the lungs and blood was achieved using temafloxacin (50 mg/kg), whereas mice remained bacteraemic even with increased dosages of ofloxacin and ciprofloxacin. In the subacute pneumonia C57B1/6-mice model, the superior efficacy of temafloxacin (compared to ofloxacin and ciprofloxacin) was also seen even when treatment was initiated at late stages of the disease (up to 96 h post-infection). The pharmacokinetics of temafloxacin account for its better efficacy against S. pneumoniae pneumonia relative to the other quinolones.