Antibiotic response and plasmid profile of bacteria isolated from a landfill

Bacteria isolates belonging to the genera Bacillus, Corynebacterium, Aeromonas, and Enterobacter were isolated from a municipal waste landfill in Durham, NC. Bacterial counts obtained with three general purpose media were log(10) colony-forming units (cfu)/g of 9.30, 9. 26, and 9.20 respectively for Plate Count Agar, Brain Heart Infusion Agar, and Nutrient Agar. Coliform count from MacConkey agar was log(10) 7.28/g sample. Isolates were generally sensitive to tetracycline and chloramphenicol (13 of 14 isolates) and generally resistant to ampicillin (9 of 9), erythromycin (10 of 14), streptomycin (8 of 14), with 3 of 14 isolates having multiple resistance to the last three antibiotics. A dose-independent growth response to ampicillin was observed for two isolates. The detection of a 22,000-bp plasmid in one but not in the second ampicillin-resistant isolate suggests more than one mechanism of antibiotic resistance.