Adaptive resistance to benzalkonium chloride, amikacin and tobramycin: the effect on susceptibility to other antimicrobials.

AIMS: To produce strains of antimicrobial-resistant Pseudomonas aeruginosa via adaptation to benzalkonium chloride, amikacin and tobramycin and to then examine the incidence, or otherwise, of cross-resistance between antibiotics and between antibiotics and benzalkonium chloride. METHODS AND RESULTS: Adaptation was obtained by progressive subculturing in subinhibitory concentrations of the antimicrobials. Pseudomonas aeruginosa NCIMB 10421 adapted to grow in high concentrations of benzalkonium chloride (BC) had lower MIC to antibiotics than the wild type, whereas Ps. aeruginosa adapted to grow in antibiotics had greater MIC to benzalkonium by a small degree. CONCLUSIONS: Adaptive resistance to BC of Ps. aeruginosa generally produced cultures with a decrease in resistance to several antibiotics. Adaptive resistance to the aminoglycosides Ak and Tm produced a low-level increase in tolerance to BC. The adaptive mechanisms of resistance appear to be different for the different types of antimicrobials used. SIGNIFICANCE AND IMPACT OF THE STUDY: The relationships between biocide and antibiotic resistance are complex. It appears, from this study, that an organism resistant to a common biocide can become sensitive to antibiotics, but the converse was not true. Could this observation be used in a strategy to alleviate antibiotic resistance?

Direct inoculation into media containing bile salts and antibiotics is unsuitable for the detection of acid/salt stressed Escherichia coli O157:H7.

The efficiency of selective enrichment broths for the recovery of low numbers of acid/salt stressed Escherichia coli O157:H7 was determined. Stressed cultures were diluted to low levels and recovered in tryptone soya broth with added bile salts, to make modified tryptone soya broth, and buffered peptone water with various combinations of antibiotic supplementation including novobiocin, acriflavine and a mixture of vancomycin, cefsulodin and cefixime (VCC) at 37 degrees C and 42 degrees C. Significantly fewer stressed cells, in some cases as little as 0.3% of the starting population, were recovered by all the selective enrichment broths containing bile salts or VCC antibiotics compared to the nonselective controls. The use of such enrichments to recover low numbers of stressed E. coli O157:H7 may result in failure to detect the organism. Parallels with salmonella methodology are made and the need for a non-selective pre-enrichment stage in E. coli O157:H7 methods discussed.

The use of an automated growth analyser to measure recovery times of single heat-injured Salmonella cells.

A new approach to the study of recovery times of single heat-injured Salmonella cells is described. It comprises the generation of a standard heat-injured culture, serial dilution of this culture to near extinction, inoculation of the serial dilutions across many microtitre plates and measurement of the subsequent recovery and growth using an automated turbidometric analyser. Lag times for individual cells were estimated from turbidity data using a model that accurately extrapolated the growth curve back to the starting inoculum level. Lag times were compared using a number of different commercially available pre-enrichment media. The most typical result was a very broad distribution of lag times at the single cell inoculum level, with many values in excess of 20 h. Even at an inoculum level 10-fold higher, lag times for some injured cells were estimated to be > 10 h. More significantly, it was found that some media recovered more injured cells than others and vice versa. Between the worst and best media there were as many as 3 log10 cycles difference in the number of cells recoverable. No trends were apparent linking choice of medium with performance. The implications of these findings, in relation to traditional and rapid methodology, are discussed.