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The discovery of communication systems regulating bacterial virulence has afforded a novel opportunity to control infectious bacteria without interfering with their growth. In this paper the authors describe the effect of subinhibitory concentrations of phenyl-lactic acid (PLA) on the pathogenicity of Staphylococcus aureus in holoxenic mice. The animals were inoculated by oral (p.o.), intranasal (i.n.), intravenous (i.v.) and intraperitoneal (i.p.) routes with S. aureus wild and PLA-treated cultures. The mice were followed up during 16 days after infection and the body weight, mortality and morbidity rate were measured every day. The microbial charge was studied by viable cell counts in lungs, spleen, intestinal mucosa and blood. The present study has proved that PLA, besides its microbicidal activity, could attenuate in subinhibitory concentrations the virulence and pathogenicity of S. aureus strains, as demonstrated by in vivo holoxenic mouse infection experimental model. Our results are accounting for the hypothesis that subinhibitory concentrations of PLA, which are not affecting the bacterial cell viability, are probably interfering with the intracellular communication and the sequential and coordinated expression of different virulence factors, altering the success of this pathogen in the colonization of sensitive hosts and the development of an infectious process.

The discovery of intra- and intercellular communication systems (quorum sensing systems) regulating bacterial virulence has afforded a novel opportunity to control infectious bacteria, without interfering with their growth. In this study, we investigated the ability of subinhibitory concentrations (sIC) of phenyl lactic acid (PLA), known to be produced by Lactobacillus probiotic strains, to attenuate the virulence and pathogenicity of Pseudomonas aeruginosa (as experimental model of intercellular bacterial communication in Gram-negative bacteria) and Staphylococcus aureus (as experimental model of intercellular bacterial communication in Gram-positive bacteria) by interfering with the coordinated expression of different virulence factors implicated in the pathogenicity of these opportunistic strains. Our results showed that sIC of PLA decreased the ability of the tested strains to adhere both to the cellular and inert substrata and induced changes in the adherence patterns as well as in the cell morphology. The sIC of PLA induced a significant decrease of sheep red blood cells haemolysins, lecithinase and caseinase and stimulated lipase and gelatinase production by Pseudomonas strains. The sIC of PLA induced an important and constant increase of the Pseudomonas growth inhibition zones diameters for all tested antibiotics, demonstrating the potential use of PLA in the design of new synergic antimicrobial associations active on multiresistant and biofilm-growing P, aeruginosa strains. The present study has proved the role of sIC of PLA released by Lactobacillus probiotic strains in the attenuation of P. aeruginosa and S. aureus virulence and pathogenicity, by interfering with different processes depending on cell density and regulated by quorum sensing (i.e. growth rate, expression of adhesion molecules and secretion of soluble, enzymatic factors) and altering the success of these pathogens in the colonization of a sensitive host and the development of an infectious process. Our results demonstrate that this probiotic soluble products could be used as a new, ecological anti-infective strategy with great therapeutic and preventive value in the biomedical field (especially in the treatment of chronic infections produced by multiresistant and biofilm forming microorganisms), but also in the management of the environmental quality, agriculture and industrial field by reducing the chemical burden delivered in the external medium and by preventing the surfaces colonization with microorganisms and the development of natural biofilms.

The purpose of this study was to evaluate the influence of different physico-chemical parameters on Escherichia coli susceptibility to ceftriaxone (CRO), cefotaxime (CTX), imipenem (IMP), and nalidixic acid (as marker for resistance by impermeability). The influence of chemical composition of culture medium was evaluated by the comparative assessment of inhibition growth diameters on different solid media: Mueller Hinton Medium (MH), Plate Count Agar Medium (PCA), MacConkey Medium (MC) and Eosin Methylen Blue Medium (EMB). In order to evaluate the differences in antibiotic susceptibility between the biofilm embedded and planktonic cells, an original, simple experimental model was used, by including the bacterial cells in an agar layer, mimicking the biofilm matrix. Our results demonstrated that the inhibition diameter zone was much larger on PCA, EMB and MC than on MH, considered as general standard medium for the antibiosusceptibility testings (CLSI). When bacterial cells were included in the agar matrix, the growth inhibition diameters obtained for different beta-lactams proved to be different of planktonic cells, i.e.: for CTX, a narrow inhibition diameter was obtained, demonstrating the low efficiency of this antibiotic in the treatment of biofilm associated infections, whereas the CRO proved the same efficiency against planktonic as well as to agar embedded bacteria. The different susceptibility results obtained for the cells embedded in the agar matrix by an adapted disk diffusion method are pleading for the necessity to assess new adapted standard methods and specific parameters in the purpose to determine the antibiotic resistance of bacterial cells isolated from biofilm associated infections.

The purpose of this study was to evaluate in vitro, by means of qualitative and quantative methods, the antimicrobial activity of some new synthesized chemical compounds previously solubilized in DMF (Dimethyl formamide). The qualitative screening of the susceptibility spectra of different microbial strains versus these compounds was performed by three adaptated diffusion methods: paper filter disk impregnation with the tested substances solutions, the disposal of tested solutions in agar wells and the spotting of tested solutions on solid medium previously inoculated with microbial suspension. The quantitative assay of the antimicrobial activity was performed by broth microdilution method in 96-well microplates in order to establish the minimal inhibitory concentration (MIC).The antimicrobial activity was tested against Gram-positive strains (Staphylococcus) aureus sp., Bacillus subtilis sp.), Gram-negative (Escherichia coli sp, Pseudomonas aeruginosa sp., Klebsiella pneumonia sp.) and fungal strains (Candida albicans sp, Aspergillus nige rsp.). All tests were performed by comparison with the reference strains: K. pneumoniae IC 13420, E. coli IC 13529, S. aureus IC 13204, P. aeruginosa IC 13202, B. subtilis IC 12488, C. albicans IC 249, A. niger IC 13534. Our results indicated that the tested compounds exhibited specific antimicrobial activity, the highest activity being noticed against planktonic fungal cells (MIC ranging from 62.5 microg to 15.6 microg), followed by P. aeruginosa (MICs from 250 microg to 31.5 microg). Only few compounds exhibited antimicrobial activity against E. coli and K. pneumoniae. Antimicrobial activity against Gram positive bacteria was noticed for the most of the tested compounds, five of them exhibiting very low MIC values (MICs from 1000 microg to 62.5 microg).

In order to survive in changing environments, bacteria possess enormous adaptive capabilities that allow them to modulate their behavior and reprogram gene expression in response to environmental cues. Vibrios are inhabitants of estuarine and fresh waters and some species are pathogenic to humans, and marine vertebrates and invertebrates. Surface attachment is believed to be essential for colonization of all of these natural environments. Vibrio (V.) parahaemolyticus, an ubiquitous marine bacterium and human pathogen, seems to be particularly adapted to growth on surfaces or in biofilms. In response to its physical environment, V. parahaemolyticus induces the expression of a large number of genes. The purpose of the present study was to investigate the influence of different physicochemical parameters (temperature and osmolarity) on the virulence factors expression in Vibrio strains using different conditions simulating environmental stress factors. Some of the tested strains displayed a decreased adherence capacity to the inert substrate under stressful conditions, and the adherence capacity on HeLa cell was generally reduced, while the soluble enzymatic factors showed only slight changes. However, it is to be noticed that the haemolysins and Kanagawa enterotoxin were better expressed at higher temperature and osmolarity, these factors probably contributing to the bacterial adaptation and survival in the extern medium of certain Vibrio species.

The virulence and resistance (R) features of 37 Aeromonas strains from diarrheal cases and 150 from the aquatic environment (isolated during cold and warm season) were tested at different incubation temperatures (4 degrees C, 28 degrees C and 37 degrees C). When incubated at 4 degrees C temperature, the Aeromonasstrains isolated during the cold season expressed the highest number of virulence factors by comparison with the strains isolated during warm season and from diarrhoeal cases, the virulence spectrum increasing simultaneously with the incubation temperature (i.e. 28 degrees C and 37 degrees C) for all strains. Mucinase was the unique virulence factor constantly present in all three categories of strains at all three incubation temperatures. The aquatic as well as clinical strains exhibited similar R levels to ampicillin and colistin, while for the other tested antibiotics, the aquatic strains generally proved higher R levels than clinical strains, excepting cephtazidime. Plasmids of molecular weights ranging between 1904-21226 bp, were isolated in 36.5% of Aeromonas strains, some of them being correlated with specific R patterns. The large virulence spectrum correlated with high R in Aeromonas strains isolated from the aquatic medium is pleading for the significant role of these bacteria in the pathogenic potential of the natural reservoir possibly implicated in human pathology.