Research Product

Culturable counts of antibiotic resistant, genetically engineered Pseudomonas fluorescens were determined on antibiotic-containing plate count agar during starvation in nutrient deficient water. Prior to starvation, colony counts obtained on all media separated into two groups. The mean of the colony counts on plate count agar with or without tetracycline (4.9 X 106 ml-1) was significantly higher than the mean colony counts on all other media tested (2.5 X 106 ml-1). After 20 days of starvation the highest mean colony counts continued to be obtained on plate count agar (7.2 X 106 ml-1) with slightly, but significantly, lower counts obtained on plate count agar containing either nalidixic acid (5.6 X 106 ml-1) or tetracycline (1.5 X 106 ml-1). In addition, a combination of nalidixic acid and tetracycline in plate count agar dramatically reduced colony counts (8.3 X 102 ml-1) after this starvation period. The addition of catalase to plate count agar containing nalidixic acid and tetracycline negated the effect caused by this combination of antibiotics. When colony counts obtained over the entire 20 day incubation were considered, the addition of MgSO4 to plate count agar containing nalidixic and tetracycline resulted in a significant increase in colony counts. Other antibiotics, added alone or in combination, did not inhibit colony formation of starved cells. Antibiotic resistant strains of P. putida and Escherichia coli also displayed sensitivity to the combination of nalidixic acid and tetracycline in plate count agar after starvation.