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The aim of this work was to study the antimicrobial activity of 395 strains of lactic acid bacteria isolated from Genestoso cheese throughout manufacture and ripening. These were: Lactobacillus (137 strains), Lactococcus (125 strains), Leuconostoc (58 strains) and Enterococcus (75 strains). After the extract had been neutralized and treated with catalase to exclude the action of hydrogen peroxide, 13 lactococci, three lactobacilli, three leuconostocs and five enterococci continued to show antimicrobial activity against one or more of the reference varieties used as indicator strains (Enterococcus faecalis CECT 481, Staphylococcus aureus CECT 240, Lactobacillus plantarum CECT 748, Clostridium tyrobutyricum CECT 4011 and Listeria monocytogenes CECT 4031). The 24 strains of lactic acid bacteria showing antimicrobial activity after excluding the action of hydrogen peroxide were identified at species level by genetic methods. The majority of these strains were isolated from the batch ripened in autumn (37.5% of strains) and the batch ripened in spring (29.2% of 15 strains) manufactured by the same cheesemaker and mainly during manufacture and the first days of ripening.

Six batches of Armada cheese were produced, one from raw milk with no added starter, another from pasteurized milk with a commercial mesophilic starter and four from pasteurized milk with experimental starters. These starters included: Lactococcus lactis subsp. lactis and L. lactis subsp. lactis biovar. diacetylactis; or the same strains plus either Enterococcus raffinosus, Leuconostoc mesenteroides subsp. dextranicum or Lactobacillus plantarum, all isolated from Armada cheese made from raw milk. The highest counts of aerobic mesophilic bacteria and populations of lactic acid bacteria for all batches were reached in the first week of ripening. These counts declined later throughout the ripening, although not at identical rates in every batch. Counts of lactobacilli were significantly higher in cheeses made from raw milk and those inoculated with the lactobacilli strain than in the other batches, over the whole ripening period. Added native starters minimized growth of Enterobacteriaceae. Cheeses made with the starter containing the Enterococcus strain had the most favourable sensory attributes throughout ripening.

Thirty-one lactic acid bacteria isolated from Armada cheese and previously selected in accordance with their technological properties, were screened for antimicrobial activity one against another. Four strains showed inhibitory activity against 14 strains when tested by well diffusion assay after the effects of organic acid and hydrogen peroxide were eliminated. Extracts of the strains did not show inhibitory activity after treatment with proteinase K, trypsin, α-chymotrypsin or pepsin. None of the strains showed antimicrobial activity against several pathogenic and spoilage reference strains. Eighteen representative strains were tested for their antibiotic resistance. None of the strains were totally susceptible to all antibiotics tested and multiple resistance was observed. Most of the tested strains were resistant to cefotoxin, oxacillin, vancomycin, teicoplanin, nitrofurantoin and trimethoprim.