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Azithromycin (AZM) is a major drug used in the treatment and prophylaxis of infections caused byChlamydia , yet no significant clinical resistance has been reported for these obligate intracellular bacteria. Nevertheless, spontaneous AZM resistance (Azmr ) arosein vitro at frequencies ranging from 3 × 10−8 to 8 × 10−10 for clonal isolates ofChlamydia caviae , which is a natural pathogen of guinea pigs. Sequencing of the unique 23S rRNA gene copy in 44 independent Azmr isolates identified single mutations at position A2058 or A2059 (Escherichia coli numbering system). While SP6 AZ1 (A2058 C) and SP6 AZ2 (A2059 C) Azmr mutants showed growth defects in cell culture and were less pathogenic in the guinea pig ocular infection model than in the parent SP6 , the three isogenicC. caviae isolates grew equally well in the animal. On the other hand, coinoculation of theC. caviae parent strain with one of the Azmr strains was detrimental for the mutant strain. This apparent lack of association between pathology and bacterial loadin vivo showed that virulence of the two Azmr mutants ofC. caviae was attenuated. While chlamydial growthin vitro reflects the ability of the bacteria to multiply in permissive cells, survival in the host is a balance between cellular multiplication and clearance by the host immune system. The obligate intracellular nature ofChlamydia may therefore limit emergence of resistancein vivo due to the strength of the immune response induced by the wild-type antibiotic-sensitive bacteria at the time of antibiotic treatment.

Impact of Azithromycin Resistance Mutations on the Virulence and Fitness of Chlamydia caviae in Guinea Pigs