Listeria monocytogenes is an opportunistic intracellular pathogen, causing serious localized and systemic infections in vertebrates. Although heterogeneity in the virulence of L. monocytogenes has been observed, a clear correlation between the level of virulence and the origin of the strain could not be established. The purpose of the present study was to investigate, by using a complex phenotypic in vitro assay, the reveal of the soluble virulence factors, as well as the expression of cell-association in 10 L. monocytogenes strains isolated from food products and 11 isolates from patients with invasive, severe listeriosis. The strains were studied for the production of enzymatic virulence factors by cultivating on media containing specific substrate for the evaluation of soluble enzymatic factors expression (pore-forming enzymes, proteases, siderophore-like production), as well as for adherence and invasion of HEp-2 cells, by Cravioto’s adapted method. The expression of the tested soluble virulence factors revealed the production of hemolysin, esculinase, and caseinase; lipase was present exclusively in strains isolated from food specimens. All tested strains (excepting two food isolates) adhered to HEp-2 cells, with a predominant diffuse-aggregative pattern, and were invasive, with modifications in the nucleus (condensations, fragmentation) and cytoplasm alterations characteristic for apoptotic cells. The adherence assay revealed the relationship between the type of adherence of tested strains and their source of isolation, the majority of them exhibiting predominantly a mixed, diffuse-aggregative pattern.

Listeria monocytogenes is an emerging bacterial foodborne pathogen responsible for listeriosis outbreaks. Frequently, listeriosis is transmitted through cured or processed meat, poultry, fish, seafood, dairy products, vegetables. This illness is characterized by septicaemia, meningitis, encephalitis and abortive disease (stillbirth or premature birth of the fetus). Listeria monocytogenes isolates from raw and processed meat were studied for the production of cell associated (adherence to HEp-2 cells) and enzymatic virulence factors, i.e.: pore forming toxins (hemolysine, lecithinase, lipase) and exoenzymes (gelatinase, amylase, caseinase, esculinase, DNase). The majority of the tested strains revealed adherece to HEp-2 cells with a predominant diffuse-aggregative pattern, as well as hemolysine, esculinase, caseinase and lipase. All L. monocytogenes strains harbored the hlyA gene. The presence of different virulence features in L. monocytogenes strains isolated from food products may explain the implication of these strains in the occurrence of severe illness.

A number of 21 B. anthracis strains isolated from 16 pustules, 2 blood cultures and 3 cerebrospinal fluids during 2000-2004 were studied for their susceptibility to antibiotics. The antibiosusceptibility testing was performed by disk diffusion method, on Mueller-Hinton agar medium. Two of the studied strains exhibited resistance to penicillins, considered until not long ago as antimicrobial agents of choice for the treatment of anthrax infection. The penicillin resistance explained the difficulties encountered during the treatment of these two cases as well as the fatal evolution in one of them. Both penicillin-resistant strains were subsequently tested, by using "in agar" antibiotic dilution method, in order to determine the minimum inhibitory concentrations (MIC) of the respective strains to penicillin G by the help of a serial antibiotic dilution from 16 microg/ml to 0.0075 microg/ml. The MIC values were 0.5 microg/ml and 4 microg/ml respectively, whereas in case of the standard B. anthracis 34F2 Sterne strain was < 0.015 microg/ml. All the 21 B. anthracis tested strains exhibited resistance to the IIIrd generation cephalosporins, as well as to TMP/STX, but were sensitive to tetracyclines and fluoroquinolones, these sensitivity aspects coming into agreement with the literature data. The strains proved to be also susceptible as follows: 13 strains to macrolides, 15 to rifampicin, 16 to chloramphenicol and all 21 to gentamycin; the last antibiotic can be used in association with fluoroquinolones in the treatment of B. anthracis infections. Fluoroquinolones (i.e. ciprofloxacin) become drugs of choice for the treatment of B. anthracis infections if early administered (within the first 24 hrs), in advance of the germ invasion into the lymph system and septicemia, preventing in this way the bacterial multiplication and production of edemathogenic and lethal toxins.