One explanation for the variability of the bacterial suspension test

Abstract

M BE RT . 2000. Disinfection kinetic studies of sodium dodecyl sulphate, benzalkonium chloride and sodium hypochlorite against Staphylococcus aureus revealed that when a higher inoculum level of Staph. aureus than normal was used (approximately 1 log higher), the efficacy of disinfection was severely attenuated. Kinetic analysis using the Hom model for experiments carried out on tests using 3 × 10 8 organisms ml −1 were unable to account for the large increase in disinfection power observed when smaller inoculum levels were used. Since the inoculum was the same in every way except for the numbers used, the large variations in the log reduction/time curves could not be explained by a variation in the resistance of the population to the biocide, as identical log reduction-time curves should have resulted. The level of disinfection achieved for a given concentration of biocide was found to be approximately linearly related to the cell number ml −1 of test solution and not to the log number. The variation observed is believed to occur due to intrinsic self-quenching of the biocide by the microbes during the course of the disinfection test. As the level of free biocide decreases, the rate of reaction decreases, giving the tails of the log reduction/time curves. Such intrinsic self-quenching could explain the large variations known to occur in the legally required disinfection suspension tests.

Listeria monocytogenes Strains Underrepresented during Selective Enrichment with an ISO Method Might Dominate during Passage through Simulated Gastric Fluid and In Vitro Infection of Caco-2 Cells

“…As a result, despite the similar inactivation rates, the populations of two competing strains in SGF could be different at each time point due to differences in their initial cell density (e.g., see the case of C5 plus 6179). Previous studies have suggested that the inoculum size can affect bacterial inactivation kinetics with lower inocula, resulting in faster inactivation (28,36). On the other hand, we showed that after 6 or 8 days of cocultivation in TSB-Y, the lower population levels of ScottA compared to C5 or PL25 populations did not lead to faster elimination of ScottA in SGF.…”

Emergence of resistance to antibacterial agents: the role of quaternary ammonium compounds—a critical review

“…Like other antibacterial agents, susceptibility to QACs may be based on the ability of QACs to kill microorganisms and/or to inhibit growth, depending on the purpose. The testing conditions can highly influence the results; thus, it is difficult to compare results between investigations and to extrapolate from laboratory to practical conditions [21][22][23][24][25]. The most influential test conditions include temperature, time of exposure, amount of organic material, growth conditions and growth phase of the microorganism to be tested.…”

“…Undoubtedly, test heterogeneity jeopardises clear interpretation of the available data [21][22][23][24][25]. Laboratory assays currently available include MIC tests for QACs, biocidal tests (i.e.…”

Evaluation of NaCl, pH, and lactic acid on the growth of Shiga toxin-producing Escherichia coli in a liquid Cheddar cheese extract

“…Considerable variability was observed in the survival of L. monocytogenes, Staph. aureus, and S. enterica between biological replicates; these results are likely the result of either differences in medium composition [i.e., redox, level of oxygen, and (or) level of nutrients] or physiological differences in the inocula (i.e., initial cell number or growth phase; Johnston et al, 2000). This variability also has affected studies done in natural cheeses (Shrestha et al, 2011a,b;Hystead et al, 2013).…”