Bacteria viable counts

Changes in the population of viable bacteria in an environment are determined by means of a viable count, and aplot of this count against time gives a dynamic picture of any pattern of change (see Fig. 11.1, curve A). The typical growth curve of a bacterial culture is constructed from data obtained in this w. The pattern of bacterial death in a lethal environment m be obtained by the same technique, when a death or mortality curve is obtained (Fig. 11.1, curve C). 

BCG Cultures of live BCG cells in liquid or on solid media 1 Bacteria centrifuged from medium 2 Resuspension in stabilizer 3 Freeze-drying Viable count induction of sensitivity to tuberculin in guinea-pigs Exclusion of virulent mycobacteria absence of excessive dermal reactivity... 

The homogeneity determination of the bacteria in the materials is performed by viable count followed by statistical evaluation of the cormts of sub-samples from the same capsule solution and of total counts of different capsules of one batch. An example for the homogeneity determination for a batch of capsules containing Enterococcus faecium is also presented in (Janning et al. 1995). 

In a parallel test with water instead of disinfectant the colony forming units (cfu) of surviving bacteria are determined and the reduction in viable counts is calculated. 

Total viable count per gram Bacteria <10 Fungi <10... 

Detection of Malo-Lactic Fermentation. It is imperative that the winemaker, to control malo-lactic fermentation, has a satisfactory method for its detection. Disappearance of malic acid is the indication of the fermentation, but the formation of lactic acid is not sufficient evidence since it might also be formed by yeast and by bacteria from other carbohydrate sources. The rate of conversion of malic acid is expected to reflect bacterial metabolism and growth. In New York State wines, Rice and Mattick (41) showed bacterial growth (as measured by viable count) to be more or less exponential to 106-107 cells/ml, preceding disappearance of malic acid. The rate of loss of malic acid is probably also exponential. Malic acid seems to disappear so slowly that its loss is not detected until a bacterial population of about 106-107 cells/ml is reached then it seems to disappear so rapidly that its complete loss is detected within a few days (41). Rice and Mattick (41) also showed a slight increase in bacterial population for a few days following this. 

Total Viable1 Anaerobes Total Viable Aerobes Total Bacteria (microscopic counts)... 

Compared with suspended (planktonic) cells, bacteria on surfaces as biofilms are invariably phe-notypically more resistant to antimicrobial agents. With biofilms, suspension tests can be modified to involve biofilms produced on small pieces of an appropriate glass or metal substrate, or on the bottom of microtitre tray wells. After being immersed in, or exposed to the disinfectant solution for the appropriate time interval, the cells from the biofilm are removed, e.g. by sonication, and resuspended in a suitable neutralizing medium. Viable counts are then performed on the resulting planktonic cells. 

Microbial limits Total viable count for aerobic bacteria and yeasts/molds. X X... 

Traditionally methods of viable counting and microscopy have been used to study phagocytosis of bacteria and subsequent survival or destruction." However, such indirect methods give an underestimate of bacterial survival within cells. It has been shown that bioluminescence acts as a convenient real-time method for monitoring survival of Bordetella bronchiseptica in vitro, whilst a dual gfp-luxABCDE operon has been used to monitor real-time replication of Staphylococcus aureus. The use of clinically important bacteria transformed with the lux cassette overcomes the problems with viable but non-culturable bacteria, as the expression of lux genes depends on the functional biochemistry of the bacteria. ""... 

Three common food borne pathogenic bacteria were transformed with plasmids carrying the lux genes in order to evaluate wet and dry surface pasteurisation of food surfaces. The work was carried out within the Bugdeath programme, an EU Framework V collaborative project to develop predictive models for the surface pasteurisation of raw food materials, based on accurate data obtained from real food samples, heated in standardised, precisely controlled conditions. Previous models, based on indirect viable counts of bacteria, have been shown to be poor predictors of bacterial inactivation and recovery during heat treatment. 

Table 2. Growth in Mixed Cultures of Pair COMPETING STRAINS s of Variant Strains Time of Amt Growth Incuba- of tion Growth of Salmonella typhimurium 4 viable Count (10 bacteria)/ml of ...

The ability of biocide-treated biofilms to utilise nutritional substrates can be determined by a direct viable count (DVC), which is measured by the effect of nalidixic acid (Lisle et al., 1999). A further method described by Holtmann and Sell (2001) for indirectly measuring the metabolic activity of biocide-treated bacteria is the determination of redox potential in the biofilm with microelectrodes. It can be shown that the increase of redox potential is correlated with a reduction in the colony count and the dehydrogenase activity. Microelectrodes are proposed as a way of monitoring the success of biocide treatments. 

One-day-old biofilms of P. aeruginosa on silicone disks were inactivated to below the detection limit ( > 5 log reduction of viable counts) after 10 min exposure to 0.1% sodium hypochlorite, or after 30 min exposure to 0.01% sodium hypochlorite, while the same degree of inactivation for planktonic bacteria was achieved in less than Imin (Takeo et al., 1994). 

Relatively few studies have included the effect of chlorine dioxide on biofilms. Characklis (1990) mentions that chlorine dioxide has been successfully used to control biofouling in several industrial environments. Walker and Morales (1997) studied the effect of chlorine dioxide on a mixed population of drinking water bacteria in a continuous culture model which was developed to simulate an industrial water system. The addition of Img/L chlorine dioxide for approximately 18 h was sufficient to reduce the viable counts of the planktonic population by 99.9%, whereas 1.5 mg/L chlorine dioxide was required to achieve a similar reduction in the biofilms, suggesting an enhanced resistance of biofilm bacteria to the biocide. There are indications that continuous disinfection of drinking water using chlorine dioxide provides a certain control of biofilm formation. In a French drinking water distribution system, the presence of chlorine dioxide allowed a limited surface colonization, while in regions where chlorine dioxide was below the detection limit, an increase in biofilm formation occurred (Servais et al., 1995). 

Yu, F. P., Pyle, B. H. and McFeters, G. A., 1993. A direct viable count method for the enumeration of attached bacteria and assessment of biofilm disinfection. Journal of Microbiological Methods 17, 167-180. 

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