Listeria monocytogenes strains isolated from dry milk samples in Mexico: occurrence and antibiotic sensitivity.

Dry milk is a particular concern in Mexico, as approximately 150,000 metric tons of dry milk are imported every year at a cost of around $250 million. Dry milk is used to make many products, most of which are dairy products widely distributed among the population covered by welfare programs. The purpose of the study described in this article was to determine the presence of Listeria spp. in imported dry milk samples in Mexico, and to determine the sensitivity of the Listeria monocytogenes isolates to different antimicrobial agents. Listeria isolates (7.8% of 550 bacterial isolates) were identified as L. monocytogenes (53.49%), L. innocua (30.23%), L. seeligeri (13.95%), and L. ivanovii (2.33%). L. monocytogenes strains isolated showed multiresistance to ampicillin, erythromycin, tetracycline, dicloxacillin, and trimethoprim-sulfamethoxazole (9%-14%). The results provide additional evidence of the emergence of multiresistant Listeria strains both in nature and in widely consumed dairy products, representing a potential threat to human health.

Isolation of Yersinia from raw meat (pork and chicken) and precooked meat (porcine tongues and sausages) collected from commercial establishments in Mexico City.

A total of 160 meat product samples were collected from commercial outlets in Mexico City to investigate the presence of different species of Yersinia by the 4 degrees C enrichment method after 1, 3, 5, and 7 days of incubation using alkaline treatment and isolating in cefsulodin-Irgasan-novobiocin and MacConkey agars with Tween 80. Overall, Yersinia spp. were isolated from 27% of the samples analyzed, whereas 40% of the raw and only 13% of the precooked samples were contaminated. Although 2,970 colonies showed Yersinia characteristics, only 706 (24%) actually corresponded to this genus: 49% were Yersinia enterocolitica, 25% Yersinia kristensenii, 15% Yersinia intermedia, 9% Yersinia frederiksenii, and 2% Yersinia aldovae; 10% corresponded to biotype 2, 2% to biotype 3, and 4% to biotype 4. The presence of Yersinia in raw and cooked meat products represents a health risk for consumers in Mexico, where further clinical studies are needed to assess the epidemiological importance of this pathogen.

Frequency of isolation of Campylobacter from roasted chicken samples from Mexico City.

The presence of Campylobacter spp. was investigated in 100 samples of roasted chicken tacos sold in well-established commercial outlets and semisettled street stands in Mexico City. From 600 colonies displaying Campylobacter morphology only 123 isolates were positive. From these isolates, 51 (41%) were identified as C. jejuni, 23 (19%) as C. coli, and 49 (40%) as other species of this genus. All of the 27 positive samples came from one location where handling practices allowed cross-contamination of the cooked product. The results indicate that these ready-to-consume products are contaminated with these bacteria, representing a potential risk for consumers, especially in establishments lacking adequate sanitary measures to prevent cross-contamination.

Occurrence and antibiotic sensitivity of Listeria monocytogenes strains isolated from oysters, fish, and estuarine water.

We analyzed the presence of Listeria spp. in oyster, fish, and seawater samples and tested isolates for antibiotic sensitivity. Listeria monocytogenes was found in 4.5% of fish samples and 8.3% of seawater samples and was not recovered from oysters. Multiresistant environmental strains were found, representing a potential threat to human health.

[Characterization of a substance which protects Klebsiella pneumoniae from the bactericidal effect of normal human serum]. / Caracterización de una sustancia que protege a Klebsiella pneumoniae del efecto bactericida del suero humano normal.

Chemical composition of a substance produced by Klebsiella pneumoniae which is accumulated in chemically defined medium was determined. This substance (the protective factor) protects the bacterium from being killed by normal human serum. Protective factor was treated with DNase, lysozyme and an alkaline protease. The two former enzymes did not affect the protective factor. On the other hand, when alkaline protease was used, the protective activity was totally lost. Results showed that the protective factor is a protein.

[Substance produced by Klebsiella pneumoniae with protects it from the bactericidal effect of normal human serum]. / Substancia producida por Klebsiella pneumoniae que la protege del efecto bactericida del suero humano normal.

A Klebsiella pneumoniae culture in Fouad's chemically defined medium, which was grown from 18 to 24 hours at 37 degrees C was prepared. Bacterial cells were eliminated by centrifugation and the supernatant was sterilized by Millipore membrane filtration. A protective activity against normal human serum bactericidal effect on K. pneumoniae K-9 and K-13 was ascribed to sterile filtrate. The protective filtrate seems to be a high molecular weight substance.