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Emergence of penicillin-resistant Streptococcus pneumoniae ocular infections
1Sid W. Richardson Ocular Microbiology Laboratory, Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.
Background:
Approximately 15-20% of Streptococcus pneumoniae clinical isolates in the United States are not susceptible to penicillin. Isolates with a minimum inhibitory concentration (MIC) of 0.12-1.0 mg/ml are intermediately resistant, and those with an MIC > 2.0 microg/ml are classified as having high-level penicillin resistance.
Purpose:
We determined the proportion of penicillin-resistant S. pneumoniae recovered from ocular and periocular infections from 1976 through 1995, compared these cases with penicillin-susceptible controls, and evaluated the susceptibility of penicillin-resistant isolates to selected cephalosporins and fluoroquinolones.
Methods:
MICs for cephalothin, ceftazidime, ciprofloxacin, and ofloxacin were determined for available isolates by the E test. We performed a case-comparison study to evaluate differences between patients with penicillin-susceptible and -nonsusceptible ocular pneumococcal infections.
Results:
Of 173 ocular isolates of S. pneumoniae isolated in the 20-year period, 12 (7%) were not susceptible to penicillin, including eight (5%) intermediate isolates and four (2%) highly resistant isolates. Penicillin-intermediate and -resistant pneumococci were recovered at a rate of none of 46 isolates from 1976 through 1980, one (4%) of 25 from 1981 through 1985, one (2%) of 51 from 1986 through 1990, and 10 (20%) of 51 from 1991 through 1995 (p < 0.0004). We found no significant differences in presenting characteristics between patients with ocular infections due to penicillin-susceptible pneumococci and those caused by nonsusceptible strains. All retested isolates with intermediate susceptibility to penicillin had a cephalothin MIC < or = 1.5 microg/ml, and all retested isolates with high-level penicillin resistance had a cephalothin MIC > or = 4 microg/ml. The ceftazidime MIC for all penicillin-resistant isolates was > or = 24 microg/ml. All penicillin-nonsusceptible isolates had MICs < or = 1.5 microg/ml for ciprofloxacin and < or = 3 microg/ml for ofloxacin.
Conclusion:
Penicillin resistance has recently emerged among ocular S. pneumoniae. Cephalothin, ciprofloxacin, and ofloxacin have good activity against some ocular isolates with intermediate penicillin susceptibility, although another agent such as vancomycin may be needed for pneumococci with high-level penicillin-resistance.
Insights
Penicillin resistance in ocular Streptococcus pneumoniae has emerged, with rates increasing significantly from 1991-1995. While some cephalosporins and fluoroquinolones show activity, high-level resistance may require alternative treatments like vancomycin.
Area of Science:
- Ophthalmology
- Microbiology
- Infectious Diseases
Background:
- Penicillin resistance in Streptococcus pneumoniae is a growing concern, affecting 15-20% of clinical isolates in the US.
- Penicillin resistance is categorized by minimum inhibitory concentration (MIC): intermediate (0.12-1.0 mg/ml) and high-level (>2.0 microg/ml).
Purpose of the Study:
- To determine the prevalence of penicillin-resistant S. pneumoniae in ocular infections from 1976-1995.
- To compare cases of penicillin-resistant S. pneumoniae with susceptible controls.
- To evaluate the susceptibility of penicillin-resistant isolates to specific cephalosporins and fluoroquinolones.
Main Methods:
- Minimum inhibitory concentrations (MICs) for cephalothin, ceftazidime, ciprofloxacin, and ofloxacin were determined using the E test.
- A case-comparison study design was employed to analyze differences between patients with penicillin-susceptible and non-susceptible ocular pneumococcal infections.
Main Results:
- Out of 173 ocular S. pneumoniae isolates, 12 (7%) demonstrated penicillin non-susceptibility (8 intermediate, 4 high-level).
- Penicillin resistance rates increased significantly over time, from 0% (1976-1980) to 20% (1991-1995).
- All penicillin-non-susceptible isolates showed susceptibility to ciprofloxacin (MIC ≤1.5 μg/ml) and ofloxacin (MIC ≤3 μg/ml).
Conclusions:
- Penicillin resistance has recently emerged in ocular S. pneumoniae isolates.
- Cephalothin, ciprofloxacin, and ofloxacin demonstrate good activity against ocular isolates with intermediate penicillin susceptibility.
- Pneumococci exhibiting high-level penicillin resistance may necessitate alternative treatments, such as vancomycin.