Maria M. Lleo

mRNA detection by reverse transcription-PCR for monitoring viability over time in an Enterococcus faecalis viable but nonculturable population maintained in a laboratory microcosm.

ABSTRACT The viable but nonculturable (VBNC) state is a survival strategy adopted by bacteria when they are exposed to hostile environmental conditions. It has been shown that VBNC forms of bacteria are no longer capable of growing on conventional bacteriological media but conserve pathogenic factors and/or genes. It is thus necessary to develop methods capable of detecting nonculturable bacteria and of establishing their viability when the microbiological quality of environments is monitored. In this study we demonstrated that a gene was expressed during the VBNC state in a low-nutrient-concentration microcosm through detection of Enterococcus faecalis pbp5 mRNA by reverse transcription-PCR over a 3-month period. The presence of mRNA correlated with metabolic activity and resuscitation capability, indicating the viability of the VBNC cells.

The Viable but Nonculturable State and Starvation Are Different Stress Responses of Enterococcus faecalis, as Determined by Proteome Analysis

The protein expression patterns of exponentially growing, starved, and viable but nonculturable (VBNC) Enterococcus faecalis cells were analyzed to establish whether differences exist between the VBNC state and other stress responses. The results indicate that the protein profile of VBNC cells differs from that of either starved or exponentially growing bacteria. This demonstrates that the VBNC state is a distinct physiological phase within the life cycle of E. faecalis, which is activated in response to multiple environmental stresses.

Resuscitation rate in different enterococcal species in the viable but non‐culturable state

Aims: The viable but non‐culturable (VBNC) state is a survival strategy adopted by bacteria when exposed to environmental stress. When in this state bacteria are no longer culturable on conventional growth media, but cells display metabolic activity and maintain pathogenicity factors/genes and, in some cases, resuscitation from the VBNC state has been shown. This state has been described for both human pathogens and faecal pollution indicators. In this study, we present evidence for entry of different enterococcal species into the VBNC state in an oligotrophic microcosm.

Cell wall chemical composition of Enterococcus faecalis in the viable but nonculturable state.

ABSTRACT The viable but nonculturable (VBNC) state is a survival mechanism adopted by many bacteria (including those of medical interest) when exposed to adverse environmental conditions. In this state bacteria lose the ability to grow in bacteriological media but maintain viability and pathogenicity and sometimes are able to revert to regular division upon restoration of normal growth conditions. The aim of this work was to analyze the biochemical composition of the cell wall ofEnterococcus faecalis in the VBNC state in comparison with exponentially growing and stationary cells. VBNC enterococcal cells appeared as slightly elongated and were endowed with a wall more resistant to mechanical disruption than dividing cells. Analysis of the peptidoglycan chemical composition showed an increase in total cross-linking, which rose from 39% in growing cells to 48% in VBNC cells. This increase was detected in oligomers of a higher order than dimers, such as trimers (24% increase), tetramers (37% increase), pentamers (65% increase), and higher oligomers (95% increase). Changes were also observed in penicillin binding proteins (PBPs), the enzymes involved in the terminal stages of peptidoglycan assembly, with PBPs 5 and 1 being prevalent, and in autolytic enzymes, with a threefold increase in the activity of latent muramidase-1 in E. faecalis in the VBNC state. Accessory wall polymers such as teichoic acid and lipoteichoic acid proved unchanged and doubled in quantity, respectively, in VBNC cells in comparison to dividing cells. It is suggested that all these changes in the cell wall of VBNC enterococci are specific to this particular physiological state. This may provide indirect confirmation of the viability of these cells.

Nonculturable Enterococcus faecalis cells are metabolically active and capable of resuming active growth.

Entry into the viable but nonculturable (VNC) state is a survival mechanism that bacteria can adopt when they find themselves in an adverse environment. When in this state, bacteria are still viable but are unable to form colonies on growth medium. The possibility of Gram-positive species entering the VNC state when environmental conditions are adverse and remaining viable and capable of resuming active growth is demonstrated for the first time in this study by using exponential-phase cultures of Enterococcus faecalis inoculated in filtered, sterilized water from Lake Garada (Italy). Over the 60-day study, the number of total cells stained with a fluorescent or counted with a Coulter Counter remained constant, while the number of cells capable of forming colonies on Tryptic Soy Agar (TSA) declined rapidly from 10(6) CFU/ml on day 0 to 10(3) CFU/ml on day 4. On day 14 no colonies could be observed when 50 ml of inoculated lake water were plated. E. faecalis cells conserved their viability while in the VNC state, as can be demonstrated by active uptake of amino acids, which are also incorporated into proteins, and by continuous detection of E. faecalis specific DNA by PCR throughout the experiment. The possibility of revival of the E. faecalis cells in the VNC state when returned to conditions supporting its cell growth has also been demonstrated. The data obtained in this study lend further support to recent criticisms of the traditional methods used to evaluate water quality based on plate counts, assessing fecal contamination indicators such as Escherichia coli and fecal streptococci.

Lipoteichoic acid as a new target for activity of antibiotics: mode of action of daptomycin (LY146032).

Daptomycin at the MIC allowed the cell mass increase of enterococcal strains and Bacillus subtilis to continue for 2 to 3 h at rates comparable to those of the controls. During this time the cell shape of the former changed to a rod configuration and that of the latter changed to long rods. In these bacteria, in which cell mass continued to increase, the MIC of daptomycin inhibited peptidoglycan synthesis by no more than 20% after 20 min of incubation and by roughly 50% after 2 h of incubation. Other macromolecules, such as DNA, RNA, and proteins, were only slightly affected. In contrast, incorporation of [14C]acetate into lipids was reduced by about 50% in the various strains after 20 min of treatment with daptomycin at the MIC. When the effect of the major lipid-containing polymers on synthesis was evaluated in detail, it was found that under conditions in which peptidoglycan and the other macromolecules mentioned above were inhibited only slightly (20%) and total lipid synthesis was inhibited by 50%, synthesis of teichoic and lipoteichoic acid was inhibited by 50 and 93%, respectively. Daptomycin was not found to enter the cytoplasm of either bacterial or mammalian cells. It bound, in the presence of calcium ions only, to whole bacterial cells, cell walls (both those that contained and those that did not contain membranes), and isolated membranes of bacterial and mammalian cells. Washing with EDTA removed daptomycin from all cells mentioned above and cell fractions except the bacterial membrane. It is concluded that lipoteichoic acid is most likely the primary target of daptomycin. Images

Modification of the Peptidoglycan of Escherichia coli in the Viable But Nonculturable State

The aim of this study was to analyse the chemical composition of peptidoglycan and the state of some of the enzymes involved in its metabolism in Escherichia coli KN126 in the viable but nonculturable (VBNC) state which is a survival strategy adopted by bacteria (including those of medical interest) when exposed to environmental stresses. When entering the VBNC state, E. coli cells miniaturised and became coccus-shaped. Analysis of peptidoglycan chemical composition, by separation in HPLC of muropeptides released by muramidase digestion of purified peptidoglycan, indicated a high degree of cross-linking, a threefold increase in unusual DAP–DAP cross-linking, an increase in muropeptides bearing covalently bound lipoprotein, and a shortening of the average length of glycan strands in comparison with dividing cells. Analysis of penicillin-binding proteins (PBPs), enzymes involved in the terminal stage of peptidoglycan assembly showed the disappearance of high-molecular-weight PBPs 1A, 1B, 2, and 3 in VBNC cells. Finally, VBNC cells displayed an autolytic capability which was far higher than that of exponentially growing cells. It is suggested that part of these alterations of peptidoglycan may be connected with the VBNC state.

Adhesion of Enterococcus faecalis in the Nonculturable State to Plankton Is the Main Mechanism Responsible for Persistence of This Bacterium in both Lake and Seawater

ABSTRACT The presence of enterococci in lake and seawater in an 18-month survey comparing molecular (PCR and quantitative PCR) and culture methods was evaluated, as well as the possibility that zooplankton could act as reservoirs for enterococci. Samples of both water and zooplankton were collected monthly from a Lake Garda site and an Adriatic Sea site. In lake water, the positive samples numbered 13 of 54 (24%) by culture and 32 of 54 (59%) when PCR was applied. In seawater, they numbered 0 of 51 by culture and 18 of 51 (35%) by PCR. Enterococci were found either totally bound to plankton or totally in water, depending on the presence or absence of plankton, respectively. These results clearly indicate that the PCR assay is a powerful tool for detecting fecal indicators and pathogens in the environment, thus providing a much more sensitive method than culture.

Molecular vs culture methods for the detection of bacterial faecal indicators in groundwater for human use

Aims:  The current standard culture methods are unable to detect nongrowing bacteria and, thus, might not be sufficient for precise monitoring of the microbiological quality of waters. The use of a molecular method such as PCR could be a valid alternative to detect bacterial faecal contamination indicators such as Escherichia coli and Enterococcus faecalis and reveal the presence of culturable and nonculturable bacterial forms.

Vancomycin resistance is maintained in enterococci in the viable but nonculturable state and after division is resumed

ABSTRACT Stressed vancomycin-resistant enterococci (VRE) can activate a survival strategy known as the viable but nonculturable (VBNC) state and are able to maintain vancomycin resistance. During restoration of division they continue to express the vancomycin resistance trait. We suggest that VBNC enterococci may constitute further reservoirs of VRE and therefore represent an additional risk for human health.