Previous studies showed that a considerable proportion of Listeria monocytogenes isolates obtained from foods carry a premature stop codon (PMSC) mutation in inlA that leads to production of a truncated and secreted InlA. To further elucidate the role these mutations play in virulence of L. monocytogenes, we created isogenic mutants, including (i) natural isolates where an inlA PMSC was reverted to a wild-type inlA allele (without a PMSC) and (ii) natural isolates where a PMSC mutation was introduced into a wild-type inlA allele; isogenic mutant sets were constructed to represent two distinct inlA PMSC mutations. Phenotypical and transcriptional analysis data showed that inlA PMSC mutations do not have a polar effect on the downstream inlB. Isogenic and natural strains carrying an inlA PMSC showed significantly reduced invasion efficiencies in Caco-2 and HepG2 cell lines as well as reduced virulence in oral guinea pig infections. Guinea pigs were also orally infected with a natural strain carrying the most common inlA PMSC mutation (vaccinated group), followed by challenge with a fully virulent L. monocytogenes strain 15 days postvaccination to probe potentially immunizing effects of exposure to L. monocytogenes with inlA PMSC mutations. Vaccinated guinea pigs showed reduced bacterial loads in internal organs and improved weight gain postchallenge, indicating reduced severity of infections in guinea pigs exposed to natural strains with inlA PMSC mutations. Our data support that (i) inlA PMSC mutations are causally associated with attenuated virulence in mammalian hosts and (ii) naturally occurring virulence-attenuated L. monocytogenes strains commonly found in food confer protective immunity.

Contributions of the alternative sigma factorB to Listeria monocytogenes infection were investigated using strains bearing null mutations in sigB, prfA, or inlA or in selected inlA or prfA promoter regions. The ⌬P4 inlA strain, which has a deletion in the B -dependent P4 inlA promoter, and the ⌬sigB strain had significantly reduced invasion efficiencies relative to that of the wild-type strain in the Caco-2 human colorectal epithelial cell line, while the invasion efficiency of a strain bearing a deletion in the partially B dependent P2 prfA promoter region did not differ from that of the wild type. The virulence of the ⌬sigB and ⌬P4 inlA strains was attenuated in intragastrically inoculated guinea pigs, with the ⌬sigB strain showing greater attenuation, while the virulence capacity of the ⌬P2 prfA strain was similar to that of the wild-type strain, suggesting that attenuation of virulence due to the ⌬sigB mutation does not result from loss of B -dependent prfA transcription. Our results show that B -dependent activation of inlA is important for cell invasion and gastrointestinal infection and suggest that B -regulated genes in addition to inlA appear to contribute to gastrointestinal infection. Interestingly, the virulence of the ⌬sigB strain was not attenuated in intravenously infected guinea pigs. We conclude that (i) L. monocytogenes B plays a critical role in invasion of human host cells, (ii) B -mediated contributions to invasion are, in part, due to direct effects on inlA transcription but not on prfA transcription, and (iii) B plays a critical role during the gastrointestinal stage of listeriosis in the guinea pig but is not important for systemic spread of the organism. Listeriosis results from consumption of food contaminated withListeria monocytogenes (40). To cause a human food-borne infection, L. monocytogenes must survive conditions encountered in the oral cavity, the stomach, and the intestine, which include exposure to lysozyme activity, gastric acidity, and bile salts (16). L. monocytogenes can invade the epithelial cells of the small intestine, then spread into mesenteric lymph nodes and disseminate systemically in the blood. Listeriosis is a relatively rare disease among healthy individuals, at least in part because the majority of L. monocytogenes organisms that enter the bloodstream are captured by the Kupffer cells in the liver and are killed by neutrophils (18). Listeriosis occurs predominantly among the immunocompromised, the elderly, and pregnant women and their fetuses (33). In susceptible individuals, L. monocytogenes can cross the bloodbrain barrier to cause central nervous system infections and the placental barrier to cause fetal infections (48). L. monocytogenes cells that escape the antimicrobial activity of the neutrophils can enter hepatocytes or endothelial cells or can be disseminated further to other tissues. Following invasion of nonphagocytic host cells, L. monocytogenes can escape from the host cell vacuole, then replicate intracellularly and spread from cell to cell (6,15...

The prognosis of dogs with untreated nasal carcinomas is poor. Treatment strategies to improve outcome should be pursued.

In 2004, the isolation of an influenza virus from racing greyhounds changed the point of reference for discussions about influenza virus in dogs. A virus isolated from greyhounds did not have its origin in a previously described human influenza virus but came from a virus with an equine history. More significantly, evidence emerged to indicate that the virus was capable of transmission from dog to dog. This virus is now referred to as canine influenza virus (CIV) and is the focus of this review. Because the history of CIV is relatively short, the impact of this virus on canine health is yet to be determined.