Abstract Films were prepared with 0.2% (w/v) gellan gum, 1% (w/v) citric pectin, 0.5% (w/v) glycerol, 5 mM CaCl2, 0.05 M Ethylenediaminetetraacetic acid (EDTA), and different concentrations of an antimicrobial activity-concentrated supernatant (AMC) obtained from fermentations of Streptococcus infantarius, which produces bacteriocin-like inhibitory substances (i.e., treatments F1, F2 and F3 with 75, 90 and 120 arbitrary units (AU)/mL, respectively). The treatments were based on a minimum-inhibitory-concentration (i.e., AMC, 90 AU/mL, plus EDTA, 0.05 M) for Escherichia coli, Staphylococcus aureus and Listeria monocytogenes growing in brain-heart-infusion medium at 35 °C. The films hindered the bacterium growth in selective media: E. coli -MacConkey, S. aureus-Baird Parker and L. monocytogenes -Oxford, during 30 days at 25 °C. Moreover, the F2 films entirely inhibited the growth of the tested bacteria in a Mexican cheese medium, in 7-day cultures at 35 °C; contrariwise, controls with no film exhibited bacterial growths in the range 107 -109 CFU/g. The physical-mechanical properties of the films were changed by the addition of EDTA-AMC, being (F2 film)/(control-film with no AMC nor EDTA): Young’s modulus (MPa), 1,394/707; Elongation at break (%), 1.9/9.3; Stress at break (MPa), 5.7/12.6; Water Vapour Permeability (10-11 g m Pa-1 s-1 m -2), 3/20, and Oxygen Permeability (10-12 g m Pa-1 s -1 m-2), 1.9/1.2.

Summary The composition of the microbiota of the Aro cheese marketed in the municipality of Teotitlan de Flores Magon, Oaxaca, Mexico, was determined, quantifying the aerobic mesophilic bacteria, psychrophilic bacteria, total coliforms, Escherichia coli, Staphylococcus aureus, Salmonella, Enterococcus, Streptococcus, lactic acid bacteria, moulds and yeasts. The results showed that the Aro cheese contained pathogenic and spoilage microorganisms as well as those of industrial importance. None of the samples analysed complied with NOM-243-SSA1-2010. Thus this cheese could transmit foodborne illnesses, due to its high content of E. coli, Staphylococcus aureus and Salmonella. The shelf life of the cheese could also be affected causing economic losses to the producers. On the other hand, microorganisms of industrial importance can be obtained (Lactobacillus and Streptococcus). It is necessary to implement quality assurance systems to guarantee the production of innocuous cheeses.

Resumen Se determinó la composición de la microbiota del queso de Aro que se comercializa en el municipio de Teotitlán de Flores Magón, Oaxaca, México. Se cuantificó el contenido de: mesófilos aerobios, psicrófilos, coliformes totales, Escherichia coli, Staphylococcus aureus, Salmonella, Enterococcus, Streptococcus, bacterias ácido lácticas, mohos y levaduras. Los resultados muestran que el queso de Aro contiene microorganismos alterantes, patógenos y de importancia industrial. Ninguna muestra analizada cumple con la NOM-243-SSA1-2010. Por lo tanto, este queso pueden ser un vehículo de transmisión de enfermedades alimentarias, por su elevado contenido de: E. coli, Staphylococcus aureus y Salmonella. La vida de anaquel del queso puede verse afectada provocando pérdidas económicas a los productores. Por otro lado, se pueden obtener microorganismos de importancia industrial (Lactobacillus y Streptococcus). Es necesario implementar sistemas se aseguramiento de la calidad que garanticen la producción de quesos inocuos.