Survival of free and alginate‐encapsulated Pseudomonas aeruginosa UG2Lr in soil treated with disinfectants

Abstract

Pseudomonas aeruginosa UG2Lr, a rifampicin‐resistant strain possessing the luxAB on a chromosomal Tn5 insert, was inoculated into soil microcosms as either free cells or encapsulated in dry alginate beads. A 100‐fold increase in cell number g‐1 dry soil was observed in microcosms inoculated with alginate‐encapsulated UG2Lr after 3 weeks incubation at 22°C compared to microcosms inoculated with free cells. After 98 d, microcosms inoculated with free UG2Lr cells contained 104 cfu g‐1 dry soil compared to 107 cfu g‐1 dry soil in microcosms inoculated with alginate‐encapsulated UG2Lr cells. The effects of disinfectants on both the free and alginate‐encapsulated UG2Lr cells were also examined. 1·0% (w/g dry soil) calcium hypochlorite, formaldehyde and Spectrum Clear Bath, were added to microcosms each week for 4 weeks. Formaldehyde killed both free and alginate‐encapsulated UG2Lr cells within 14 d after only two amendments. Calcium hypochlorite reduced free UG2Lr cell numbers 10‐fold 2 d after initial application; however, the introduced population recovered and was unaffected by subsequent treatments at 7, 14 and 21 d. Alginate‐encapsulated UG2Lr cells were not affected by calcium hypochlorite treatment. Spectrum Clear Bath did not kill either free or alginate‐encapsulated UG2Lr cells in soil. Alginate encapsulation improved survival of introduced bacteria in soil except in the presence of formaldehyde. Killing genetically‐engineered bacteria in soil may be difficult unless a powerful disinfectant such as formaldehyde is used or the genetically‐engineered micro‐organism is allowed to become non‐viable over time.