Microbiology applied to food analysis: Fecal pollution detection
Fecal – oral pollution arises from the lack of hygiene and food manipulation that results in direct or crossed contamination 1. The consumption of dairy products and non -pasteurized juices, elaborated meat and insufficiently cooked, raw fruits and vegetables, are one of the most common sources of food transmitted infections. Among them bacteria such as Bacillus SP, Clostridium perfringens, Escherichia coli, Shigella SP, Yersinia SP and Staphylococcus aureus, have been found in these foods.
The main objective of the Microbiology Laboratory is to apply precise methods of bacterial phenotypic indexes involved in the development of infections. Therefore, microorganisms are used that serve as indicators of fecal contamination, which live in conditions similar to that of intestinal pathogens and develop extinctly. Among the most used indicators are fecal coliforms represented by four genres of the Enterobacteriaceae family: Citrobacteria, Enterobacter, Escherichia and Klebsiella, whose group of grammnegative bacteria, aerobic and optional anaerobias ferments lactose and produce indol.
Determination methods are mentioned for some bacteria. Biochemical tests such as decarboxylation where the formation of amines that raise the pH with a violet color turn and the use of citrate as a source of carbon and nitrogen for its metabolism are used for enterobacteria.
Enteroiveive Escherichia is the main bacteria that indicates a possible fecal contamination in food and has a relationship with the shigella genus methods are used from the positive EC broth tubes, which are sown in selective media such as Mac Conkey or agar.Methylene Blue Eosina. Subsequently, IMVIC biochemical tests are performed that positively give indole production in the presence of red coloration;Methyl red, a red coloration;In Voges –proskauer an appearance of a pink color and in citrate is observed turbiebad in the middle.
Suspicious isolates of being Shigella, are cultivated in a non -selective medium such as Kligler Iron Agar (AHK) or Triple Sugar Iron (AHTA), which are useful means to detect insulation. Also, monovalent anti -sauceer tests and sheet agglutination are applied. On the other hand, for the identification of Salmonella in food samples, they are grown in buffered peton water and inoculate in the Rappaport broth and muller broth, to be grown in Xilosa Agar plate Lysine Desicolate (XLD) (XLD). Subsequently, biochemical tests are performed that work positive to the TSI agar, the flute peak;Urea agar with the liberation of ammonium and red from phenol to cherry;Indol reaction in the formation of a red ring and in sulphite bismute agr with the presence of greenish colonies.
Other biochemical tests are the urea medium to identify ureasa producing microorganisms (Klebiella) giving as a positive result a red coloration and lying iron (ahl) for certain strains of E. coli with an alkaline reaction (purple color), shigellas are negative lysine.
Finally, coliform bacteria are useful components for microbiological criteria and indicate fecal pollution in food samples, where a series of biochemical methods and tests are applied which allow phenotypic identification of enterobacteria. However, it is worth emphasizing the importance of hygiene in food cooking and conservation in order to avoid infectious diseases and guarantee food quality.
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