Effects of different concentrations of sodium azide and chloramphenicol on the preservation of raw milk samples

portuguesA conservacao das amostras de leite destinadas para analises microbiologicas pela Rede Brasileira de Laboratorios de Controle da Qualidade do Leite requer adicao de conservantes para a preservacao da microbiota existente desde o momento da coleta ate as analises. O numero de microrganismos pode apresentar alteracoes decorrentes do principio ativo e concentracao do conservante, e ainda entre as interacoes conservante, tempo de incubacao e temperatura de acondicionamento. O objetivo deste trabalho foi avaliar o potencial de conservacao de diferentes concentracoes de azida sodica e cloranfenicol sobre a vida util analitica de amostras de leite. Foram selecionadas duas fazendas, sendo uma com baixa contagem bacteriana e outra com alta contagem bacteriana. O leite foi fracionado em frascos estereis e testado nas seguintes condicoes: pastilhas com a concentracao usual de azida sodica e cloranfenicol, com dupla concentracao, com tripla concentracao e sem a adicao do conservante. As amostras foram incubadas por quatorze dias a 3 ± 1 °C, 6 ± 1 °C e 9 ± 1 °C, e analisadas diariamente por citometria de fluxo para a determinacao da contagem bacteriana. A tripla concentracao do conservante demonstrou maior conservacao, possibilitando o aumento da viabilidade analitica das amostras sem alteracao nos resultados. EnglishThe preservation of milk samples for microbiological analyses by the Brazilian Network of Milk Quality Control Laboratories requires the addition of preservatives to maintain the microbiota from the time of sample collection to the moment of analysis. The number of microorganisms can change as a result of the active ingredients and concentration of the preservative, as well as due to interactions between the preservatives, incubation time, and packaging temperature. The objective of this research was to evaluate the conservation potential of different concentrations of sodium azide and chloramphenicol on the analytical shelf life of milk samples. Two farms were selected, one with a low bacterial count and one with a high bacterial count. The milk was dispensed into sterile vials and tested after the addition of the usual concentrations of sodium azide and chloramphenicol, doubled concentrations, tripled concentrations, and as a control, without preservatives. The samples were incubated at 3 ± 1 °C, 6 ± 1 °C, and 9 ± 1 °C for 14 days and analyzed daily for their bacterial count by flow cytometry. The tripled preservative concentrations improved conservation, increasing the timespan of the analytical viability of the samples without altering the results.