Vancomycin-tolerance among clinical isolates of Streptococcus pneumoniae in Mississippi during 1999-2001

In recent years, infection with Streptococcus pneumoniae has been a serious worldwide health concern. Antimicrobial-resistant S pneumoniae is increasing in incidence worldwide, posing a potentially serious threat. Resistance to beta-lactams, macrolides, and trimethoprim-sulfamethoxazole represents a major problem in the treatment of pneumococcal infections

Our laboratory conducted a survey of local resistance patterns in S pneumoniae. Clinical isolates from two separate respiratory seasons were collected from representative geographic areas in Mississippi (totaling 28 hospitals) and were tested for antimicrobial resistance to penicillin, amoxicillin, ceftriaxone, cefuroxime, azithromycin, clindamycin, tetracycline, trimethoprim-sulfamethoxazole, levofloxacin, gatifloxacin, moxifloxacin, and vancomycin using reference methods. Vancomycin-tolerant strains of S pneumoniae were initially identified as those in which the vancomycin MIC was 0.5 microg/mL. Strain tolerance was confirmed by time kill studies

For the 1999-2000 respiratory season, 318 isolates were available for testing; for 2001-2002, 166 isolates were available. Of the 484 total isolates tested, two isolates were identified as having increased tolerance to vancomycin. A greater than 2 log10 difference in viability between the tolerant isolates and the nontolerant isolates of S pneumoniae was observed in time kill studies

Two vancomycin-tolerant isolates of S pneumoniae were identified and characterized. Antibiotic tolerance is defined as the ability of bacteria to survive but not proliferate in the presence of an antibacterial agent. Tolerance to vancomycin is particularly significant when the incidence of penicillin tolerance or resistance is high. In addition, tolerance to vancomycin is not detected by routine in vitro susceptibility testing.