The growth and survivability of Streptococcus pneumoniae clinical isolates subjected to various environmental conditions

Historically, it has been hypothesized that environmental stress would favor the survival of antibiotic susceptible bacteria over resistant ones; however, there is little direct evidence to support this theory. Clinical isolates of S. pneumoniae were chosen and categorized as: penicillin susceptible, quinolone susceptible (PSQS, n = 3); penicillin resistant, quinolone susceptible (PRQS, n = 3); and penicillin resistant, quinolone resistant (PRQR, n = 5). Baseline growth of each isolate was measured by optical density for 24 h. The resulting optical density curves were compared to those obtained for the same isolates subjected to changes in environmental conditions, such as various temperature, pH, and diluted media. In addition, each isolate was inoculated onto cotton fiber disks, held at room temperature, and the recoverable CFU measured over 144 h. In comparison to controls grown under ideal conditions, the density of PSQS isolates was significantly lower than PRQR isolates after 24 h for the following conditions (p < 0.01): incubation at 40°C (1.3 log10 lower); at pH 6.5 (1.6 log10 lower); and in limited nutrient conditions (1.36 log10 lower). When inoculated onto cotton fiber disks, the PRQR isolates decreased an average of 5.0 log10 after 72 h as compared to controls. In contrast, PSQS isolates decreased an average of 8.1 log10 (p < 0.01). Results of this study support the concept that antibiotic resistant isolates may not be at a competitive disadvantage in comparison to susceptible isolates when subjected to some adverse environmental conditions.