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Unlike most other bacteria, many Listeria strains do not grow well in the minimal media described so far in the literature. Among the minimal media tested, a chemically defined medium modified from that of Premaratne and co-workers was found to support the best growth of Listeria spp. The promoting effect was due to the incorporation of several indispensable vitamins and growth factors.

The proteins induced by heat and cold shock in Listeria monocytogenes (pathogenic for humans) and L. innocua (nonpathogenic) strains were analyzed by two-dimensional (2-D) electrophoresis with the help of a computerized 2-D gel analysis system. Heat (49 degrees C) and cold (4 degrees C) shock repressed roughly half the number of proteins synthesized at normal temperature (25 degrees C) and decreased the level of numerous other proteins. Conversely, the synthesis of a great number of proteins was enhanced and novel proteins appeared upon temperature stress. There were more proteins induced in the L. monocytogenes strain than in the L. innocua strain. Each stress induced a set of specific proteins. There was overlap between these sets of proteins induced by heat and cold shock. Furthermore, a number of heat or cold shock proteins were found to be induced in both Listeria species and by both heat and cold shock in both species. The induction by heat shock was more intense than that by cold shock. The most strongly induced common stress protein of Listeria had a molecular mass of 17.6 kDa and an isoelectric point of 5.1.

The cellular proteins of 29 Listeria strains belonging to different species and serotypes were analysed by two-dimensional (2-D) electrophoresis with the help of a computerized 2-D gel analysis system. The protein patterns were similar among strains within a Listeria species, but were different from one species to another. The comparative analysis of these protein maps enabled us to find specific proteins and to determine the genetic relatedness among Listeria spp. strains. The cluster analysis based on protein mapping showed a division between species and a clear separation of L. monocytogenes from other Listeria species. Inside L. monocytogenes the strains were divided into two main clusters in correlation with flagellar antigenic structures; this is in concordance with the results that have been found on the basis of multilocus enzyme electromorphs or DNA-restriction patterns. This technique enabled us to subtype the strains sharing the same serovar or the same lysovar. Because of its independence and high discriminating capacity, the 2-D protein mapping technique might provide a powerful tool for the identification and classification of Listeria strains.

Abstract: Ruminants fed contaminated forage may shed Listeria monocytogenes in their faeces, and prolonged low daily doses of L. monocytogenes could cause listerial infection [Maijala, R., Lyytikainen, O., Autio, T., Aalto, T., Haavisto, L., Honkanen-Buzalski, T., 2001. Exposure of Listeria monocytogenes within an epidemic caused by butter in Finland. Int. J. Food Microbiol. 70, 97–109]. To compare listerial infection following single or repeated doses and the contamination of the environment with the excreted bacteria, ewes were orally inoculated with either 104, 106 or 1010 cfu L. monocytogenes once, or daily for 10 days. Serological responses were monitored with indirect ELISAs using recombinant listeriolysin O (LLO), internalin A (InlA) and internalin A-related protein (IrpA). The 24 inoculated animals displayed no symptoms, except for a transient hyperthermia in two animals given 1010 cfu. One ewe died on day 9 after non-listerial mastitis followed by listerial septicaemia. L. monocytogenes was recovered from day 1 post-inoculation until day 17 from the faeces of ewes inoculated with 106 or 1010 cfu. No antibodies were detected in ewes given 104 or 106 cfu. Anti-LLO and anti-IrpA antibodies were detected from day 15 in animals inoculated with 1010 cfu, and this strengthened the conclusion that these long-lasting shedders were infected but asymptomatic carriers. An anti-InlA response was detected only at a very low level. These results suggest that repeated daily doses are no more effective than a single dose in causing infection in ewes. [Copyright &y& Elsevier]