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Background    Streptococcus pneumoniae ,  Streptococcus pyogenes  and  Haemophilus influenzae  are bacteria present in the nasopharynx as part of normal flora. The ecological equilibrium in the nasopharynx can be disrupted by the presence of antibiotics.    Methodology/Principal Findings  A computerized two-compartment pharmacodynamic model was used to explore β-lactam effects on the evolution over time of a bacterial load containing common pharyngeal isolates by simulating free serum concentrations obtained with amoxicillin (AMX) 875 mg tid, amoxicillin/clavulanic acid (AMC) 875/125 mg tid and cefditoren (CDN) 400 mg bid regimens over 24 h. Strains and MICs (µg/ml) of AMX, AMC and CDN were:  S. pyogenes  (0.03, 0.03 and 0.015),  S. pneumoniae  (2, 2 and 0.25), a β-lactamase positive  H. influenzae  (BL + ; >16, 2 and 0.06) and a β-lactamase positive AMC-resistant  H. influenzae  (BLPACR, >16, 8 and 0.06). Mixture of identical 1∶1∶1∶1 volumes of each bacterial suspension were prepared yielding an inocula of ≈4×10 6  cfu/ml. Antibiotic concentrations were measured both in bacterial and in bacteria-free antibiotic simulations. β-lactamase production decreased AMX concentrations and  f T >MIC  against  S. pneumoniae  (from 43.2% to 17.7%) or  S. pyogenes  (from 99.9% to 24.9%), and eradication was precluded. The presence of clavulanic acid countered this effect of co-pathogenicity, and  S. pyogenes  (but not BL +  and  S. pneumoniae ) was eradicated. Resistance of CDN to TEM β-lactamase avoided this co-pathogenicity effect, and CDN eradicated  S. pyogenes  and  H. influenzae  strains ( f T >MIC  >58%), and reduced in 94%  S. pneumoniae  counts ( f T >MIC  ≈25%).    Conclusions/Significance  Co-pathogenicity seems to be gradual since clavulanic acid countered this effect for strains very susceptible to AMX as  S. pyogenes  but not for strains with AMX MIC values in the limit of susceptibility as  S. pneumoniae . There is a potential therapeutic advantage for β-lactamase resistant cephalosporins with high activity against streptococci.

β-Lactam Effects on Mixed Cultures of Common Respiratory Isolates as an Approach to Treatment Effects on Nasopharyngeal Bacterial Population Dynamics