Antibiotic Resistance Patterns of Clinical Isolates of Serratia marcescens

Summary

Objective: Assess antibiotic resistance patterns of clinical Serratia marcescens isolates using disk diffusion and MIC (agar dilution).
Sample: 102 strains from three centers (62 MUSC, 22 Emory, 18 M. D. Anderson) collected June–December 1973.
Key result: Cephalothin resistance universal; ampicillin and tetracycline resistance very high; gentamicin, nalidixic acid, sulfisoxazole, and chloramphenicol most effective relatively.
Pigmented vs nonpigmented: pigmented strains show similar patterns with some higher resistance to certain drugs.
Implications: Resistance varies by hospital; R factors contribute transferable resistance; continuous surveillance recommended.

Disk sensitivity testing: standard single-disc method on Müller–Hinton agar using antibiotics: streptomycin (10 μg), sulfisoxazole (0.25 mg), tetracycline (30 μg), cephalothin (30 μg), kanamycin (30 μg), ampicillin (10 μg), nalidixic ext{ acid} (30 μg), gentamicin (10 μg), carbenicillin (50 μg), colistin (10 μg), tobramycin (10 μg), and chloramphenicol (30 μg).
MIC (agar dilution): using Steers inocula-replicating apparatus; inoculum ≈ 5\times 10^{4} bacteria; incubation 18\ {
m h} at 37^{\circ}\text{C}; MIC = lowest concentration preventing visible growth.
Definitions: MIC determined for each drug; susceptibility patterns summarized across centers and in relation to pigmented status.

Cephalothin: 100\% resistant.
Ampicillin: 93\% resistant.
Tetracycline: 93\% resistant.
Colistin: 57\% resistant.
Carbenicillin: 39\% resistant.
Streptomycin: 34\% resistant.
Tobramycin: 26\% resistant.
Kanamycin: 22\% resistant.
Chloramphenicol: 16\% resistant.
Sulfisoxazole: 15\% resistant.
Gentamicin: 14\% resistant.
Nalidixic acid: 8\% resistant.
Multidrug resistance: 56/102 isolates resistant to ≥5 drugs; 14/102 resistant to ≥8; 5/102 resistant to all 12.
Center-specific patterns (disk tests):
- M. D. Anderson: more frequently resistant to nearly all drugs except colistin and cephalothin.
- Emory: more resistant to colistin, kanamycin, carbenicillin, sulfisoxazole, gentamicin, and streptomycin than MUSC.
- MUSC: more susceptible overall than Emory to most drugs, except cephalothin, tobramycin, and chloramphenicol.

Pigmented isolates: 11 strains; all pigmented resistant to cephalothin, ampicillin, and tetracycline.
All pigmented strains: susceptible to chloramphenicol, kanamycin, nalidixic ext{ acid}, sulfisoxazole, and gentamicin.
Pigmented strains also: 8/11 resistant to streptomycin, 8/11 resistant to colistin, 5/11 resistant to tobramycin.
Nonpigmented strains: 65% had MICs of streptomycin in the range 5-20\ \,\text{µg/mL} (most) versus pigmented strains showing higher MICs (20–40 \ µg/mL for streptomycin).

Gentamicin, tobramycin, and chloramphenicol: >90\% inhibition at concentrations within therapeutically achievable serum levels.
Nalidixic acid: ≥90\% inhibition at concentrations achievable in urine.
Cephalothin and colistin: failed to inhibit isolates at achievable serum concentrations.
MICs for ampicillin, tetracycline, and cephalothin were higher in strains resistant to >5 drugs, indicating broader resistance.
Pigmented vs nonpigmented MIC trends largely similar except as noted for streptomycin.

Overall resistance pattern varied by center; patterns were characteristic for each hospital with respect to several drugs, except ampicillin, tetracycline, and cephalothin which remained universally or near-universally resistant across centers.
Implication: hospital-specific selective pressures influence resistance profiles.

Reach of R factors: transferable resistance reported previously; present in multi-resistant strains in this study; ongoing work to characterize plasmids and their role in resistance augmentation.
Clinical implications: polymyxins and cephalosporins least effective in vitro; gentamicin, kanamycin, chloramphenicol, and nalidixic acid most effective in vitro; ampicillin, cephalothin, tetracycline, and colistin largely ineffective.
Public health note: resistance can vary over time and between hospitals; periodic surveillance is essential.

High-level: Serratia marcescens shows substantial multidrug resistance with hospital- and pigment-status-associated patterns; certain drugs (gentamicin, tobramycin, chloramphenicol, nalidixic acid) remain comparatively effective at achievable concentrations, while cephalothin and colistin are largely ineffective; ongoing surveillance and understanding of transferable resistance factors are crucial to guide therapy.