M. Hinton

Organic Acids: Chemistry, Antibacterial Activity and Practical Applications

Publisher Summary This chapter presents the chemistry and antimicrobial activity of the saturated straight-chain monocarboxylic acids, and reference is made, where appropriate, to derivatives of this group— for example, unsaturated (cinnamic, sorbic), hydroxylic (citric. lactic), phenolic (benzoic, cinnamic, salicylic) and multicarboxylic (azelaic, citric, succinic) acids. Organic acids are distinguished from other acids by the functional group COOH to which an organic group or a hydrogen atom may be attached. Common names used to describe this group of organic compounds include fatty, volatile fatty, lipophilic, weak, or carboxylic acids. There is no standard protocol for studying the antimicrobial activity of organic acids and many reports give only the pH value of the solution under test without indicating the concentration of the acid, thus making evaluation of the results difficult or impossible. In addition, it is difficult to compare the activity of different acids because it is influenced by the physical chemistry of the microbial species, the growth conditions, and the phase of growth. The chapter discusses two aspects of the use of organic acids: in animal husbandry as animal feed additives and in abattoirs and food-processing plants where they may be used in controlling microbial contamination of carcass meat.

Antibacterial activity of formic and propionic acids in the diet of hens on salmonellas in the crop

1. The inclusion of formic and propionic acids in the form of Bio-Add to the food of hens made no difference to the pH of the intestinal tract, but resulted in higher concentrations of these acids in the contents of the crop and gizzard. 2. Organic acids in the crop contents were bactericidal for Salmonella serotype Enteritidis PT4 in vitro, and also caused sub-lethal damage because fewer cells were recovered on selective salmonella media (brilliant green phenol red agar) than on non-selective media (nutrient agar). 3. Inclusion of Bio-Add in the food at 12g/kg may reduce the number of lactic acid-producing bacteria in the crop, and hence the amount of naturally produced organic acids.

Effect of changes in processing to improve hygiene control on contamination of poultry carcasses with campylobacter.

Examination of neck skin and caecal samples taken at a commercial processing plant from 15 randomly chosen poultry flocks showed that all flocks were contaminated initially with thermophilic Campylobacter spp., even in the apparent absence of caecal carriage. During processing, numbers of campylobacter on skin samples were reduced by between 10 and 1000-fold. To improve hygiene control generally, chlorinated-water sprays were used to limit microbial contamination on equipment and working surfaces. In addition, chlorine concentrations in process water were increased and any unnecessary carcass contact surfaces in the processing plant were removed. When comparing flocks before and after the changes, it was found that numbers of campylobacter on packaged carcasses were significantly lower after the changes had been made (P 0.001). In practice, however, the reduction would be likely to have little impact on consumer exposure to campylobacter infection.

Use of a marker organism in poultry processing to identify sites of cross‐contamination and evaluate possible control measures

1. Nine different sites at a poultry processing plant were selected in the course of a hazard analysis to investigate the degree of microbial cross-contamination that could occur during processing and the effectiveness of possible control measures. 2. At each site, carcases, equipment or working surfaces were inoculated with a non-pathogenic strain of nalidixic acid-resistant Escherichia coli K12; transmission of the organism among carcases being processed was followed qualitatively and, where appropriate, quantitatively. 3. The degree of cross-contamination and the extent to which it could be controlled by the proposed measures varied from one site to another.

Recommended Assay for Treatment of Chicks to Prevent Salmonella Colonization by ‘Competitive Exclusion’

An assay is described for evaluating live-culture treatment material that may be given orally to chicks to prevent intestinal colonization by non-host-specific salmonellae. Both pre-treated and control chicks are challenged with ca 104 salmonellae/chick, using a strain bearing an antibiotic resistance marker. Chicks are examined 5 d after challenge to determine both the proportion of positive birds in treated and control groups and the levels of Salmonella in the caeca of infected individuals. The efficacy of the treatment is determined by calculation of values for Infection Factor and Protection Factor.

Microbiological survey of five poultry processing plants in the UK

1. Neck skin samples were taken from chickens and turkeys at all the main stages of processing to monitor changes in total viable count (TVC) and counts of coliforms and pseudomonads. 2. Processing reduced TVC by up to 100-fold. Geometric mean counts after packaging were log10 4.4 to 5.3 CFU/g whilst corresponding counts of coliforms were 2.7 to 3.8 CFU/g. 3. Increases in mean TVC or coliforms as a result of either defeathering or evisceration did not exceed 0.6 log. 4. Pseudomonads represented only a minor fraction of the initial microflora of the bird and were often reduced by scalding to a figure which could not be detected by direct plating of samples; however, subsequent contamination resulted in means between log10 2.9 and 4.0 CFU/g for packaged carcases. 5. Although Staphylococcus aureus was readily isolated from defeathering equipment, mean counts from defeathered carcases were always below log10 3.0 CFU/g.

Evaluation of the influence of supplementing the diet with mannose or palm kernel meal on salmonella colonisation in poultry

1. The dietary inclusion of 15 and 25 g/kg mannose was associated with a reduction in the numbers of Salmonella enteritidis (PT4) in the caecal contents of chicks challenged by the food. The same benefit was not recorded for S. infantis, possibly because this strain, unlike S. enteritidis PT4, lacked mannose-sensitive fimbriae. 2. The addition of 25 g/kg palm kernel meal (PKM), but not 20 g/kg desiccated coconut, to the food reduced the degree of salmonella colonisation in the intestinal tract of broiler chicks given diets contaminated with S. kedougou or S. enteritidis from the day of their arrival from the hatchery. 3. The beneficial effect of PKM was also demonstrated at an inclusion rate of 5 g/kg and was similar for preparations with a particle size of either < 150 microns or < 300 microns. 4. Day-old birds challenged orally with S. enteritidis and given food supplemented with 25 g/kg PKM, became clear of infection by 3 weeks of age while birds given unsupplemented food remained infected. 5. These preliminary results suggest that the inclusion of PKM, which contains inter alia, oligosaccharides containing mannose, in the diet of chicks may reduce the extent to which the intestine is contaminated with salmonellas.

Effects of fleece soiling and skinning method on the microbiology of sheep carcases

The fleece of sheep becomes soiled primarily on the abdomen and on the legs. A five-category scoring scale for soiling of the fleece was defined, with score 1 being clean and dry and score 5 being wet and heavily soiled with faecal material. Twenty sheep with each fleece score were slaughtered and dressed on a commercial ‘inverted’ slaughter line. Swab samples were taken from the shoulder and the abdomen of the carcases immediately after skinning, and total viable counts and the numbers of Enterobacteriaceae were determined. The condition of the fleece significantly affected the microbial load on these parts of the dressed carcase, with the carcases derived from sheep with increasingly dirty fleeces carrying up to 1000 times more microorganisms, and a higher proportion of the carcases being contaminated with Enterobacteriaceae. Modifications to the dressing procedure were made to try to reduce carcase contamination at the shoulder. The application of bulldog clips to prevent direct contact between the fleece and the underlying muscles resulted in higher counts on carcases with a fleece score of 3, probably owing to contamination from the hands of the slaughterman and the problems with ensuring that the clips were in place before contamination had occurred. Delaying the skinning of the shoulder until later in the pelt removal process achieved worthwhile reductions in the contamination of the shoulder of carcases with a fleece score of 4, although the total viable counts were still higher than carcases with a score of 3.

The biotyping of Escherichia coli isolated from healthy farm animals

A total of 2973 Escherichia coli, isolated from six different groups of animals, were examined for their ability to ferment adonitol, dulcitol, raffinose, rhamnose and sorbose in solid media. Twenty-nine fermentation patterns were recorded although 2443 (82%) of the E. coli belonged to seven of the 32 possible biotypes. Ninety-six O-serotypes were identified within the 2973 E. coli. The number of O-serotypes represented in the 15 most common biotypes ranged from three to 15. Serotypes O8 and O9 were found most commonly in the different groups of animals and several biotypes amongst these two O-serotypes were identified in two or more groups of the animals. The ability of the E. coli to metabolize aesculin, ornithine, salicin and sucrose was also assessed. These test proved less reproducible and were not included in the primary biotyping scheme although their use allowed the enumeration of additional biotypes. The application of biotyping to the study of the ecology of drug-resistant strains of E. coli in five situations is briefly presented.

Effect of short‐chain fatty acids on the size of enteric bacteria

J.L. THOMPSON AND M. HINTON. 1996. The size of Escherichia coli and Salmonella enteritidis PT4 cells was measured using both transmission electron micrographs and image analysis. Incubation in the presence of formic and propionic acids resulted in larger cells, possibly as a result of DNA inhibition, with no apparent damage to the cell membranes. Bacteria incubated with propionic acid were more resistant to shrinkage after fixation, possibly as a result of altered phospholipid and fatty acid composition.