Antibiotic resistance and biofilm formation of Acinetobacter baumannii isolated from high-risk effluent water in tertiary hospitals in South Africa

Abstract

Discharge of drug-resistant, biofilm-forming pathogens from hospital effluent water into municipal wastewater treatment plants poses a public health concern. This study examined the relationship between antibiotic resistance levels and biofilm formation of Acinetobacter baumannii strains isolated from hospital effluents. Antibiotic susceptibility of 71 A. baumannii isolates was evaluated by the Kirby-Bauer disk diffusion method. Minimum inhibitory concentrations (MICs) were determined by the agar dilution method, while the minimum biofilm eradication concentration (MBEC) was determined by the broth dilution method. Genotyping was performed for plasmid DNA. Biofilm formation was evaluated by the microtitre plate method and was quantified using crystal violet. A P-value of <0.05 was regarded as statistically significant in all tests. Extensively drug-resistant (XDR) strains made up 58% of the isolates, while multidrug-resistant (MDR) and pandrug-resistant (PDR) strains made up 50% of the isolates from final effluent. The MBEC of ciprofloxacin increased by 255-fold, while that of ceftazidime was as high as 63-1310-fold compared with their respective MICs. Isolates were classified into four plasmid pattern groups with no association between biofilm formation and plasmid type (P=0.0921). The degree of biofilm formation was independent of the level of antibiotic resistance, although MDR, XDR and PDR isolates produced significant biofilm biomass (P=0.2580). These results suggest that hospital effluent is a potential source of MDR biofilm-forming A. baumannii strains. Appropriate treatment and disposal of effluents are essential to prevent the presence of drug-resistant pathogens in wastewater.