Differential response of sessile and planktonic bacterial populations following exposure to antimicrobial treatment

[摘要] ENGLISH ABSTRACT:The ability of biofilms to resist antimicrobial treatment, when planktonic microbes cannot, isof not only fundamental scientific interest, but also a concern in industrial and medical fields.The inability to control biofouling of water distribution networks and products, as well asrecurrent infections of implanted medical devices, is not only costly, but also potentiallylethal. Several mechanisms whereby biofilms are able to evade antibiotic and biocidal agentshave been proposed and investigated, but no universally relevant characteristic has beenidentified. .Initial investigation, involving BacLightTh! LIVEIDEAD viability probes, epifluorescencemicroscopy and image analysis into the ability of natural biofilm and planktonic populations,.cultured in situ in a cooling tower, to survive treatment with a commercial biocide was notconclusive. Subsequent laboratory experimentation with a bacterial isolate from the coolingtower water revealed that the ability of attached biofilms to resist antimicrobial treatmentexceeded that of planktonic cells shed from the biofilm. The reduced ability of suspendedcells to survive antimicrobial treatment was not statistically significant, compared to that ofthe biofilm (P = 0.05). This is in contrast to the wealth of literature published on the subjectof biofilm antimicrobial resistanceThe dilution rate in the flowcells in which biofilms were cultivated was more than 100 timeshigher than the maximum specific growth rate of the test organism. Nevertheless, there wastypically more than I x 108 cells/ml in the effluent, suggesting that a metabolically active,rapidly dividing layer of cells existed at the biofilm bulk-liquid interface, from wheredaughter cells continuously detached. Treatment with an antimicrobial agent resulted in asignificant reduction in the viability and number of cells detached from the biofilm,suggesting that this metabolically active layer of the biofilm was more sensitive toantimicrobial treatment, possibly due to a higher specific growth rate. Antimicrobialresistance was shown to be affected by the growth rate for planktonic bacterial populations,with an increased ability to survive, correlated with a decrease in specific growth rate. Thissupports the contention that growth rate plays a role in the susceptibility of the active layer.The bacterial cells in the layers closest to the attachment surface of the biofilm has frequentlybeen shown to be slow growing, due to nutrient and oxygen limitation, while the outer biofilmlayer is more susceptible to unfavourable environmental conditions. It is possible that suchdifferentiation, which results in a responsive outer biofilm layer, provides a mechanism forthe protection of the cells in the deeper layers, and thus survival over time.The results presented here support several hypotheses put forth in literature to account for theincreased resistance of biofilms towards antimicrobial agents. Future work will include aninvestigation into changes in the patterns of gene expression when a bacteria becomesattached to a surface, upon subsequent release from the biofilm, and the influence this has onthe ability to resist antimicrobial treatment.

[发布机构] Stellenbosch University