Evidence of a dormant but infective state of the fish pathogen Pasteurella piscicida in seawater and sediment

The Viable but Non-Culturable (VBNC) State, a Poorly Explored Aspect of Beneficial Bacteria

Many bacteria have the ability to survive in challenging environments; however, they cannot all grow on standard culture media, a phenomenon known as the viable but non-culturable (VBNC) state. Bacteria commonly enter the VBNC state under nutrient-poor environments or under stressful conditions. This review explores the concept of the VBNC state, providing insights into the beneficial bacteria known to employ this strategy. The investigation covers different chemical and physical factors that can induce the latency state, cell features, and gene expression observed in cells in the VBNC state. The review also covers the significance and applications of beneficial bacteria, methods of evaluating bacterial viability, the ability of bacteria to persist in environments associated with higher organisms, and the factors that facilitate the return to the culturable state. Knowledge about beneficial bacteria capable of entering the VBNC state remains limited; however, beneficial bacteria in this state could face adverse environmental conditions and return to a culturable state when the conditions become suitable and continue to exert their beneficial effects. Likewise, this unique feature positions them as potential candidates for healthcare applications, such as the use of probiotic bacteria to enhance human health, applications in industrial microbiology for the production of prebiotics and functional foods, and in the beer and wine industry. Moreover, their use in formulations to increase crop yields and for bacterial bioremediation offers an alternative pathway to harness their beneficial attributes.

A Secreted NlpC/P60 Endopeptidase from Photobacterium damselae subsp. piscicida Cleaves the Peptidoglycan of Potentially Competing Bacteria

Peptidoglycan (PG) is a major component of the bacterial cell wall, forming a mesh-like structure enwrapping the bacteria that is essential for maintaining structural integrity and providing support for anchoring other components of the cell envelope. PG biogenesis is highly dynamic and requires multiple enzymes, including several hydrolases that cleave glycosidic or amide bonds in the PG. This work describes the structural and functional characterization of an NlpC/P60-containing peptidase from Photobacterium damselae subsp. piscicida (Phdp), a Gram-negative bacterium that causes high mortality of warm-water marine fish with great impact for the aquaculture industry. PnpA ( PhotobacteriumNlpC-like protein A) has a four-domain structure with a hydrophobic and narrow access to the catalytic center and specificity for the γ-d-glutamyl-meso-diaminopimelic acid bond. However, PnpA does not cleave the PG of Phdp or PG of several Gram-negative and Gram-positive bacterial species. Interestingly, it is secreted by the Phdp type II secretion system and degrades the PG of Vibrio anguillarum and Vibrio vulnificus This suggests that PnpA is used by Phdp to gain an advantage over bacteria that compete for the same resources or to obtain nutrients in nutrient-scarce environments. Comparison of the muropeptide composition of PG susceptible and resistant to the catalytic activity of PnpA showed that the global content of muropeptides is similar, suggesting that susceptibility to PnpA is determined by the three-dimensional organization of the muropeptides in the PG.IMPORTANCE Peptidoglycan (PG) is a major component of the bacterial cell wall formed by long chains of two alternating sugars interconnected by short peptides, generating a mesh-like structure that enwraps the bacterial cell. Although PG provides structural integrity and support for anchoring other components of the cell envelope, it is constantly being remodeled through the action of specific enzymes that cleave or join its components. Here, it is shown that Photobacterium damselae subsp. piscicida, a bacterium that causes high mortality in warm-water marine fish, produces PnpA, an enzyme that is secreted into the environment and is able to cleave the PG of potentially competing bacteria, either to gain a competitive advantage and/or to obtain nutrients. The specificity of PnpA for the PG of some bacteria and its inability to cleave others may be explained by differences in the structure of the PG mesh and not by different muropeptide composition.

Vaccines and immune protection of principal Mediterranean marine fish species

This review describes and summarizes the knowledge on established and experimental vaccines developed against viral and bacterial pathologies affecting the most important farmed marine finfish species present in the Mediterranean area, namely European seabass Dicentrarchus labrax, sea bream Sparus aurata, turbot Psetta maxima and meagre Argyrosomus regius. The diseases that have been recorded in seabass, sea bream and meagre are caused by bacteria Vibrio anguillarum, Photobacterium damselae, Tenacibaculum maritimum as well as by viruses such as Viral Encephalopathy and Retinopathy/Viral Nervous Necrosis and Lymphocystic disease. The main pathologies of turbot are instead bacteriosis provoked by Tenacibaculum maritimum, Aeromonas sp. and Vibrio anguillarum, and virosis by viral hemorrhagic septicaemia virus. Some vaccines have been optimized and are now regularly available for the majority of the above-mentioned pathogens. A measurable immune protection has been conferred principally against Vibrio anguillarum, Photobacterium damselae sub. piscicida and VER/VNN.

Genomic analysis of the marine fish pathogen Photobacterium damselae subsp. piscicida: Insertion sequences proliferation is associated with chromosomal reorganisations and rampant gene decay

Photobacterium damselae subsp. piscicida (Pdp) is an intracellular fish pathogen that causes photobacteriosis, a disease proven deadly in farmed fish worldwide. This work focuses on the analysis of genome sequences, chromosomes structure and gene contents of two strains from Sparus aurata (DI21) and Solea senegalensis (L091106-03H), isolated on the Spanish Atlantic coast. The comparative genomic analysis revealed that DI21 and L091106-03H share 98% of their genomes, including two virulence plasmids: pPHDP70 encoding siderophore piscibactin synthesis and pPHDP10 encoding the apoptotic toxin AIP56. Both genomes harbour a surprisingly large number of IS elements accounting for 12-17% of the total genome, representing an IS density of 0.15 elements per kb, one of the highest IS density values in a bacterial pathogen. This massive proliferation of ISs is responsible for the generation of a high number of pseudogenes that caused extensive loss of biological functions. Pseudogene formation is one of the main features of Pdp genome that explains most of the ecological and phenotypic differences with respect to its sibling subspecies P. damselae subsp. damselae and to other Vibrionaceae. Evidence was also found proving the existence of two chromosomal configurations depending on the origin of the strains: an European and an Asian/American types of genome organisation, reinforcing the idea of the existence of two geographically-linked clonal lineages in Pdp. In short, our study suggests that the host-dependent lifestyle of Pdp allowed massive IS proliferation and gene decay processes, which are major evolutionary forces in the shaping of the Pdp genome.

Fate and Persistence of a Pathogenic NDM-1-Positive Escherichia coli Strain in Anaerobic and Aerobic Sludge Microcosms

The presence of emerging biological pollutants in treated wastewater effluents has gained attention due to increased interest in water reuse. To evaluate the effectiveness of the removal of such contaminants by the conventional wastewater treatment process, the fate and decay kinetics of NDM-1-positive Escherichia coli strain PI7 and its plasmid-encoded antibiotic resistance genes (ARGs) were assessed in microcosms of anaerobic and aerobic sludge. Results showed that E. coli PI7 decayed at a significantly lower rate under anaerobic conditions. Approximate half-lives were 32.4 ± 1.4 h and 5.9 ± 0.9 h in the anaerobic and aerobic microcosms, respectively. In the aerobic microcosms, after 72 h of operation, E. coli PI7 remained detectable, but no further decay was observed. Instead, 1 in every 10,000 E. coli cells was identified to be recalcitrant to decay and persist indefinitely in the sludge. ARGs associated with the E. coli PI7 strain were detected to have transferred to other native microorganisms in the sludge or were released to the liquid fraction upon host decay. Extracellular DNA quickly degraded in the liquid fraction of the aerobic sludge. In contrast, no DNA decay was detected in the anaerobic sludge water matrix throughout the 24-h sampling period. This study suggests an increased likelihood of environmental dispersion of ARGs associated with anaerobically treated wastewater effluents and highlights the potential importance of persister cells in the dissemination of E. coli in the environment during reuse events of treated wastewater.

IMPORTANCE This study examines the decay kinetics of a pathogenic and antibiotic resistant strain of Escherichia coli in microcosms simulating biological treatment units of aerobic and anaerobic sludge. The results of this study point at a significantly prolonged persistence of the E. coli and the associated antibiotic resistance gene in the anaerobic sludge. However, horizontal transfer of the plasmid encoding the antibiotic resistance gene was detected in the aerobic sludge by a cultivation method. A subpopulation of persister E. coli cells was also detected in the aerobic sludge. The findings of this study suggest potential areas of concern arising from pathogenic and antibiotic-resistant E. coli during both anaerobic and aerobic sludge treatment processes.

Hypometabolism as the ultimate defence in stress response: how the comparative approach helps understanding of medically relevant questions

First conceptualized from breath-hold diving mammals, later recognized as the ultimate cell autonomous survival strategy in anoxia-tolerant vertebrates and burrowing or hibernating rodents, hypometabolism is typically recruited by resilient organisms to withstand and recover from otherwise life-threatening hazards. Through the coordinated down-regulation of biosynthetic, proliferative and electrogenic expenditures at times when little ATP can be generated, a metabolism turned 'down to the pilot light' allows the re-balancing of energy demand with supply at a greatly suppressed level in response to noxious exogenous stimuli or seasonal endogenous cues. A unifying hallmark of stress-tolerant organisms, the adaptation effectively prevents lethal depletion of ATP, thus delineating a marked contrast with susceptible species. Along with disengaged macromolecular syntheses, attenuated transmembrane ion shuttling and PO₂ -conforming respiration rates, the metabolic slowdown in tolerant species usually culminates in a non-cycling, quiescent phenotype. However, such a reprogramming also occurs in leading human pathophysiologies. Ranging from microbial infections through ischaemia-driven infarcts to solid malignancies, cells involved in these disorders may again invoke hypometabolism to endure conditions non-permissive for growth. At the same time, their reduced activities underlie the frequent development of a general resistance to therapeutic interventions. On the other hand, a controlled induction of hypometabolic and/or hypothermic states by pharmacological means has recently stimulated intense research aimed at improved organ preservation and patient survival in situations requiring acutely administered critical care. The current review article therefore presents an up-to-date survey of concepts and applications of a coordinated and reversibly down-regulated metabolic rate as the ultimate defence in stress responses.

Dynamics of piscine francisellosis differs amongst tilapia species (Oreochromis spp.) in a controlled challenge with Francisella noatunensis subsp. orientalis

A 25-week immersion challenge was conducted exposing Oreochromis mossambicus, Oreochromis aureus and Oreochromis urolepis hornorum to Francisella noatunensis subsp. orientalis (Fno). Two populations were compared for each fish species; 'resident fish' were defined as fish maintained in tanks since week 0 of challenge, whereas 'naïve fish' were defined as fish added to tanks once temperature in water reached <26 °C at 21 weeks post-challenge. Fno genome equivalents (GEs) in water were similar in all treatments 1 h post-challenge; however, significantly lower Fno GEs were detected 2 weeks post-challenge in all tanks, and the only treatment with detectable Fno GE after 4 weeks of challenge were the O. mossambicus tanks. Twenty-one weeks post-challenge, naïve fish were stocked with 'resident' cohorts. Over a 4-week period, mortalities occurred consistently only in O. mossambicus naïve cohorts. Overall presence of granulomas in spleen of survivors was similar (>55%) in all resident populations; however, in naïve populations, only O. mossambicus presented granulomas. Similarly, only O. mossambicus presented viable Fno in the spleen of survivors, and Fno GEs were only detected in O. mossambicus, and in resident O. aureus. In conclusion, the results of this study suggest different susceptibility of tilapia species to piscine francisellosis.

Survival behaviour and virulence of the fish pathogen Vibrio ordalii in seawater microcosms

Vibrio ordalii, the causative agent of atypical vibriosis, is a Gram-negative, motile, rod-shaped bacterium that severely affects the salmonid aquaculture industry. V. ordalii has been biochemically, antigenically and genetically characterized. However, studies on the survival behaviour of this bacterium in aquatic environments are scarce, and there is no information regarding its disease transmission and infectious abilities outside of the fish host or regarding water as a possible reservoir. The present study investigated the survival behaviour of V. ordalii Vo-LM-06 and Vo-LM-18 in sterile and non-sterile seawater microcosms. After a year in sterile seawater without nutrients, 1% of both V. ordalii strains survived (~10(3) colony-forming units ml(-1)), and long-term maintenance did not affect bacterial biochemical or genetic properties. Additionally, V. ordalii maintained for 60 d in sterile seawater remained infective in rainbow trout Oncorhynchus mykiss. However, after 2 d of natural seawater exposure, this bacterium became non-culturable, indicating that autochthonous microbiota may play an important role in survival. Recuperation assays that added fresh medium to non-sterile microcosms did not favour V. ordalii recovery on solid media. Our results contribute towards a better understanding of V. ordalii survival behaviour in seawater ecosystems.

New aspects in the biology of Photobacterium damselae subsp. piscicida: pili, motility and adherence to solid surfaces

We describe for the first time the presence of pilus-like structures on the surface of Photobacterium damselae subsp. piscicida (Phdp). The hint to this discovery was the ability of one strain to hemagglutinate human erythrocytes. Further analysis of several Phdp strains ultrastructure by electron microscopy revealed the presence of long, thin fibers, similar to pili of other Gram-negative bacteria. These appendages were also observed and photographed by scanning, transmission electron microscopy and immunofluorescence. Although this fish pathogen has been described as non-motile, all strains tested exhibit twitching motility, a flagella-independent type IV-dependent form of bacterial translocation over surfaces. As far as we are aware, the movement of Phdp bacteria on semi-solid or solid surfaces has not been described previously. Moreover, we speculate that Phdp twitching motility may be involved in biofilm formation. Microscopic examination of Phdp biofilms by microscopy revealed that Phdp biofilm architecture display extensive cellular chaining and also bacterial mortality during biofilm formation in vitro. Based on our results, standardized analyses of Phdp surface appendages, biofilms, motility and their impact on Phdp survival, ecology and pathobiology are now feasible.

The importance of the viable but non-culturable state in human bacterial pathogens

Many bacterial species have been found to exist in a viable but non-culturable (VBNC) state since its discovery in 1982. VBNC cells are characterized by a loss of culturability on routine agar, which impairs their detection by conventional plate count techniques. This leads to an underestimation of total viable cells in environmental or clinical samples, and thus poses a risk to public health. In this review, we present recent findings on the VBNC state of human bacterial pathogens. The characteristics of VBNC cells, including the similarities and differences to viable, culturable cells and dead cells, and different detection methods are discussed. Exposure to various stresses can induce the VBNC state, and VBNC cells may be resuscitated back to culturable cells under suitable stimuli. The conditions that trigger the induction of the VBNC state and resuscitation from it are summarized and the mechanisms underlying these two processes are discussed. Last but not least, the significance of VBNC cells and their potential influence on human health are also reviewed.

Thirty years of viable but nonculturable state research: unsolved molecular mechanisms

Viable but nonculturable (VBNC) cells were recognized 30 years ago; and despite decades of research on the topic, most results are disperse and apparently incongruous. Since its description, a huge controversy arose regarding the ecological significance of this state: is it a degradation process without real significance for bacterial life cycles or is it an adaptive strategy of bacteria to cope with stressful conditions? In order to solve the molecular mechanisms of VBNC state induction and resuscitation, researchers in the field must be aware and overcome common issues delaying research progress. In this review, we discuss the intrinsic characteristic features of VBNC cells, the first clues on what is behind the VBNC state's induction, the models proposed for their resuscitation and the current methods to prove not only that cells are in VBNC state but also that they are able to resuscitate.

Monitoring the survival of fish-pathogenic Francisella in water microcosms

In this report, the survival behaviour of fish pathogenic Francisella in water microcosms was investigated under laboratory conditions. Two isolates of Francisella noatunensis (NCIMB14265(T) and PQ 1106), from fish held in seawater and freshwater, were inoculated into natural (nonsterile) and sterile sea- and freshwater microcosms, respectively, and monitored under different temperature conditions (4, 8 and 12 °C) over a period of 60 days. The culturability of the strains was inversely related to the water temperature. Strain NCIMB14265(T) was found to survive longer in seawater than PQ 1106 held in freshwater at equivalent temperatures. The survival of both strains was higher in sterile than in nonsterile microcosms. These results were confirmed by quantitative PCR analysis targeting the succinate dehydrogenase (sdhA) gene. A cell viability assay coupled with FISH analyses showed that F. noatunensis cells enter a viable but not culturable (VBNC) state after a period in water. However, although metabolically active, the VBNC cells were not pathogenic to cod (Gadhus morhua) following an intraperitoneal challenge, under the conditions tested. The data presented contribute to a better understanding of the behaviour of F. noatunensis in natural seawater and freshwater environments, and show the need for further investigation of the role of VBNC cells in the environmental transmission of this pathogen.

Recent findings on the viable but nonculturable state in pathogenic bacteria

Many bacteria, including a variety of important human pathogens, are known to respond to various environmental stresses by entry into a novel physiological state, where the cells remain viable, but are no longer culturable on standard laboratory media. On resuscitation from this 'viable but nonculturable' (VBNC) state, the cells regain culturability and the renewed ability to cause infection. It is likely that the VBNC state is a survival strategy, although several interesting alternative explanations have been suggested. This review describes the VBNC state, the various chemical and physical factors known to induce cells into this state, the cellular traits and gene expression exhibited by VBNC cells, their antibiotic resistance, retention of virulence and ability to attach and persist in the environment, and factors that have been found to allow resuscitation of VBNC cells. Along with simple reversal of the inducing stresses, a variety of interesting chemical and biological factors have been shown to allow resuscitation, including extracellular resuscitation-promoting proteins, a novel quorum-sensing system (AI-3) and interactions with amoeba. Finally, the central role of catalase in the VBNC response of some bacteria, including its genetic regulation, is described.

Development of a multiplex PCR assay for Photobacterium damselae subsp. piscicida identification in fish samples

A multiplex polymerase chain reaction protocol for the detection of Photobacterium damselae and subspecies piscicida and damselae discrimination, with internal amplification control, was developed. Assay specificity was assessed by testing 19 target and 25 non-target pure cultures. The detection limit was 500 fg, corresponding to 100 genome equivalents. The optimized protocol was also prevalidated with spleen, kidney and blood samples from infected and uninfected sea bass, without any culture step, and it can be proposed as a valid alternative to culture standard methods for the rapid and specific diagnosis of photobacteriosis in fish.

Invasion and survival of Photobacterium damselae subsp. piscicida in non-phagocytic cells of gilthead sea bream, Sparus aurata L

Fluorescence microscopy and gentamicin protection assays were used to investigate the ability of four Photobacterium damselae subsp. pisicida (Phdp) strains to adhere to and to invade the fish epithelial cell line, SAF-1, derived from Sparus aurata. All strains tested were detected intracellularly using both techniques, although internalization levels varied among strains. Treatment with cytochalasin D and experiments carried out at 4 degrees C demonstrated that a functional host cell cytoskeleton and active cell metabolism are necessary for bacterial internalization. Intracellular bacteria were detected for up to 7 days with a round morphology and were stained with DAPI, indicating that some bacterial cells may remain viable inside SAF-1 cells. Our in vitro findings indicate that Phdp are capable of adhering, entering and surviving within the non-phagocytic epithelial cell line SAF-1, which may be important for persistence and establishment of a carrier state in S. aurata.

Survival of Vibrio fluvialis in seawater under starvation conditions

The viability of Vibrio fluvialis in seawater microcosms, with and without sediment was investigated. The strain survived as culturable bacteria for at least 1 year and the expression of its virulence factors was maintained. In microcosms containing sediment Vibrio fluvialis was more stable. Viable but nonculturable (VBNC) cells of Vibrio fluvialis were able to resuscitate to the culturable state up to 6 years of incubation in marine sediment. These cells recuperate their initial biochemical characteristics after 3 months of incubation in marine broth. Amplified 16S ribosomal DNA (rDNA) restriction analysis (ARDRA) was used to confirm that it is the same strain of Vibrio fluvialis which resists in all microcosms during a long period of time.

Use of microcosms to determine the survival of the fish pathogen Tenacibaculum maritimum in seawater

The survival of the fish pathogen Tenacibaculum maritimum in different seawater microcosms was investigated during 160 days. The persistence of culturable cells was greater in sterile than in natural seawater. Standard plate counts showed that T. maritimum survived in sterile seawater for more than 5 months at concentration around 10(3) cfu ml(-1). However, T. maritimum proved to be very labile in non-sterile seawater, rendering culturable cells no longer than 5 days. These results were confirmed when DNA-based methods were applied. Regardless of the microcosms used, epifluorescence microscopy counts remained at about 10(6) cells ml(-1) throughout the experiment, even though we can not distinguish T. maritimum in the case of non-sterile microcosms. Resuscitation assays with addition of fresh medium to non-sterile microcosms did not favour the recovery of T. maritimum on solid media. Although morphological changes from filamentous to spheres were observed after 3 days in the non-sterile microcosms, in the case of the sterile microcosms this change was observed at the sixth day. The biochemical, physiological, serological and genetic characteristics were unaffected in the sterile microcosms. The overall results contribute to a better understanding of the behaviour of T. maritimum in natural seawater and suggest that the aquatic bacterial population play an important role in the survival of this fish pathogen.

Atypical growth of Renibacterium salmoninarum in subclinical infections

Two growth types of Renibacterium salmoninarum were isolated from subclinically infected rainbow trout, one producing the smooth colonies typical of R. salmoninarum and the other forming a thin film on the surface of the agar with no separate colonies. The atypical growth was present on kidney disease medium agar in primary cultures of the kidney but not on selective kidney disease medium (SKDM). Fluorescent antibody staining of the fresh isolate and polymerase chain reaction amplification were the most reliable techniques to identify the atypical growth of R. salmoninarum. The condition was reversible, with growth reverting from atypical to the smooth colony form in experimentally infected rainbow trout and under laboratory conditions. There was no mortality, or any clinical signs of bacterial kidney disease (BKD) in the fish challenged with the atypical growth, although small numbers of smooth colonies of R. salmoninarum were isolated from 8% of these fish. The atypical growth reported here may explain some of the failures of culture, when SKDM agar alone is used for the detection of BKD in subclinically infected fish.

Mycobacterium avium enters a state of metabolic dormancy in response to starvation

Members of the Mycobacterium avium complex (MAC) exhibit a highly effective and biphasic response to starvation, losing less than 90% viability after 2 years in deionized water. During the first adaptive phase of 4-7 days, the bacilli exhibit a burst of lipid catabolism, alteration of mycolate modifications, loss of catalase and urease activities, and a decline in sensitivity to antibiotics. There is also a decline in the protein level of alanine tRNA synthetase (AlaS), and an increase in ribonuclease E (Rne) levels. During the following persistence phase, the bacilli become metabolically dormant. However, with return of nutrients, the cells rapidly respond with increased activity, as determined by reduction of a tetrazolium dye. The primary reservoir for MAC is natural and municipal water, and the metabolic dormancy may be analogous to that of other aquatic organisms, such as vibrio. The organized metabolic shutdown that environmental mycobacteria utilize to survive starvation may have evolved into the host-specific dormancy mechanisms of Mycobacterium tuberculosis.

Influences of temperature, salinity and starvation on the motility and chemotactic response of Vibrio anguillarum

The role of growth factors for the motility and chemotaxis of the fish pathogenVibrio anguillarumwas determined. Cells ofV. anguillarumwere chemotactic to serine in the temperature range 5–25 °C and in 0·8–2·7 % NaCl. The chemotactic response was significantly higher at 25 °C than at 5 or 15 °C. Growth in medium with 1·5 % NaCl gave a higher response than growth with 3 % NaCl; when the salinity of the chemotaxis buffer was raised, the chemotactic response was reduced. The role of starvation was also studied;V. anguillarumshowed a high chemotactic response after starvation for 2 and 8 days. Motility and chemotaxis are important virulence factors for this bacterium. Not only was the ability to perform chemotactic motility maintained after starvation, but also it was shown that starvation does not interfere with the ability of the organism to cause infection in rainbow trout after a bath challenge. The swimming speed was reduced at lower temperatures. Within the range of salinity and starvation studied, the motile cells swam with the same velocity, indicating thatV. anguillarumunder all the examined conditions has a functional flagellum and rotates it with constant speed. Phenamil, a specific inhibitor of Na+-driven flagella, reduced the motility of both starved and non-starved cells ofV. anguillarumindicating that, in both cases, a Na+motive force drives the flagellum.

Behavior of an Aeromonas hydrophila aroA live vaccine in water microcosms

Genetically modified auxotrophic mutants of different fish pathogens have been used as live vaccines in laboratory experiments, but the behavior of the strains after release into aquatic ecosystems has not been characterized. We previously constructed and characterized an aroA mutant of Aeromonas hydrophila and studied the protection afforded by this mutant as a live vaccine in rainbow trout. In this work, we describe the survival of this strain in aquatic microcosms prepared from fish water tanks. The aroA mutant disappeared rapidly in nonfiltered, nonautoclaved fish tank water, declining below detection levels after 15 days, suggesting an inhibitory effect of the autochthonous microflora of the water. When the aroA strain was used to inoculate sterilized water, its culturability was lower than that of wild-type strain A. hydrophila AG2; after long periods of incubation, aroA cells were able to enter a viable but nonculturable state. Entry into this nonculturable state was accompanied by changes in the cell morphology from rods to spheres, but the cells appeared to remain potentially viable, as assessed by the preservation of cell membrane integrity. Supplementation of the culture medium with sodium pyruvate favored the culturability and resuscitation of the two A. hydrophila strains at low temperatures (6 and 16 degrees C). These results contribute to a better understanding of the behavior of the aroA strain in natural environments and suggest that the inactivation of the aroA gene may be beneficial for the safety of this live vaccine for aquacultures.

Adoption of the transiently non-culturable state — a bacterial survival strategy?

Microbial culturability can be ephemeral. Cells are not merely either dead or alive but can adopt physiological states in which they appear to be (transiently) non-culturable under conditions in which they are known normally to be able to grow and divide. The reacquisition of culturability from such states is referred to as resuscitation. We here develop the idea that this "transient non-culturability" is a consequence of a special survival strategy, and summarise the morphological, physiological and genetic evidence underpinning such behaviour and its adaptive significance.

Development, validation, and application of PCR primers for detection of tetracycline efflux genes of gram-negative bacteria

Phylogenetic analysis of tetracycline resistance genes, which confer resistance due to the efflux of tetracycline from the cell catalyzed by drug:H(+) antiport and share a common structure with 12 transmembrane segments (12-TMS), suggested the monophyletic origin of these genes. With a high degree of confidence, this tet subcluster unifies 11 genes encoding tet efflux pumps and includes tet(A), tet(B), tet(C), tet(D), tet(E), tet(G), tet(H), tet(J), tet(Y), tet(Z), and tet(30). Phylogeny-aided alignments were used to design a set of PCR primers for detection, retrieval, and sequence analysis of the corresponding gene fragments from a variety of bacterial and environmental sources. After rigorous validation with the characterized control tet templates, this primer set was used to determine the genotype of the corresponding tetracycline resistance genes in total DNA of swine feed and feces and in the lagoons and groundwater underlying two large swine production facilities known to be impacted by waste seepage. The compounded tet fingerprint of animal feed was found to be tetCDEHZ, while the corresponding fingerprint of total intestinal microbiota was tetBCGHYZ. Interestingly, the tet fingerprints in geographically distant waste lagoons were identical (tetBCEHYZ) and were similar to the fecal fingerprint at the third location mentioned above. Despite the sporadic detection of chlortetracycline in waste lagoons, no auxiliary diversity of tet genes in comparison with the fecal diversity could be detected, suggesting that the tet pool is generated mainly in the gut of tetracycline-fed animals, with a negligible contribution from selection imposed by tetracycline that is released into the environment. The tet efflux genes were found to be percolating into the underlying groundwater and could be detected as far as 250 m downstream from the lagoons. With yet another family of tet genes, this study confirmed our earlier findings that the antibiotic resistance gene pool generated in animal production systems may be mobile and persistent in the environment with the potential to enter the food chain.

Survival of Mannheimia (Pasteurella) haemolytica in tracheobronchial washings of sheep and cattle

The growth, morphology and long-term survival of a representative isolate of Mannheimia haemolytica serotypes A1 and A2 were monitored in ovine and bovine tracheobronchial washings. Both strains survived for at least 244 days in ovine tracheobronchial washings and 156 days in bovine tracheobronchial washings. The addition of fresh washings at these times prompted an increase for serotype A2 but no change in viability for serotype A1 in ovine tracheobronchial washings and an increase for both serotypes in bovine tracheobronchial washings. When growth and survival was compared using tracheobronchial washings from ruminant and non-ruminant species there was a trend towards longer survival in ruminant fluids.Long-term survival was associated with temporary or permanent change from normal size colonies to 'micro-colonies' on sheep blood agar. Subculture allowed reversion to normal colony morphology. Analysis showed these micro-colonies to consist of chains of elongated bacteria. M. haemolytica serotype A2 was more robust in its ability to withstand nutrient deprivation for long periods of time. These survival mechanisms may have important implications for pathogenesis.

16S rRNA gene sequence analysis of Photobacterium damselae and nested PCR method for rapid detection of the causative agent of fish pasteurellosis

The causative agent of fish pasteurellosis, the organism formerly known as Pasteurella piscicida, has been reclassified as Photobacterium damselae subsp. piscicida on the basis of 16S rRNA gene sequence comparisons and chromosomal DNA-DNA hybridization data; thus, this organism belongs to the same species as Photobacterium damselae subsp. damselae (formerly Vibrio damselae). Since reassignment of P. damselae subsp. piscicida was based on only two strains, one objective of the present work was to confirm the taxonomic position of this fish pathogen by sequencing the 16S rRNA genes of 26 strains having different geographic and host origins. In addition, a nested PCR protocol for detection of P. damselae based on 16S rRNA was developed. This PCR protocol was validated by testing 35 target and 24 nontarget pure cultures, and the detection limits obtained ranged from 1 pg to 10 fg of DNA (200 to 20 cells). A similar level of sensitivity was observed when the PCR protocol was applied to fish tissues spiked with bacteria. The PCR approach described in this paper allows detection of the pathogen in mixed plate cultures obtained from asymptomatic fish suspected to be carriers of P. damselae subsp. piscicida, in which growth of this bacterium cannot be visualized. Our results indicate that the selective primers which we designed represent a powerful tool for sensitive and specific detection of fish pasteurellosis.

Production of viable cultures of Flavobacterium psychrophilum: approach and control

Although the fish pathogen Flavobacterium psychrophilum is a major source of concern in salmonid hatcheries, few studies have been conducted on its pathogenicity. Difficulties are often experienced when trying to control or quantify standard procedures for in vitro culture of the bacterium. Plate enumeration and counting chamber enumeration combined with epifluorescent microscopy with fluorescent dyes determined that no more than 25% of the bacterial cells present in the cultures were able to produce colonies on agar media. This was strongly dependent upon different medium components. Tryptone-enriched Anacker and Ordal medium proved more suitable than tryptone-yeast extract-salts with skimmed milk. Adding horse serum and trace elements in controlled proportions offered the most reproducible results. Viable but nonculturable forms were apparently not responsible for the difficulties in production of F. psychrophilum, but the cells were highly susceptible to osmotic conditions. Improvements in the media and careful handling of the bacteria in isotonic suspension media resulted in predictable production of viable bacteria and allowed an absorbance/colony-forming-units relation curve to be established.

Survival of fish-virulent strains of Photobacterium damselae subsp. damselae in seawater under starvation conditions

The survival of fish-virulent strains of Photobacterium damselae subsp. damselae in seawater microcosms, with and without sediment, was investigated. The strains survived as culturable bacteria at 14 and 22 degrees C for at least 1 year, and infectivity for fish was maintained. At 5 degrees C, cells lost culturability on solid media, but this was recovered when the temperature was increased to 22 degrees C. Finally, morphological changes in the bacterium (rod to coccus), and production of vesicles and extracellular material were observed during the time of starvation. The overall results suggest that seawater and sediment can act as reservoirs for these virulent strains.

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.

Flow cytometry and cell sorting of heterogeneous microbial populations: the importance of single-cell analyses

The most fundamental questions such as whether a cell is alive, in the sense of being able to divide or to form a colony, may sometimes be very hard to answer, since even axenic microbial cultures are extremely heterogeneous. Analyses that seek to correlate such things as viability, which is a property of an individual cell, with macroscopic measurements of culture variables such as ATP content, respiratory activity, and so on, must inevitably fail. It is therefore necessary to make physiological measurements on individual cells. Flow cytometry is such a technique, which allows one to analyze cells rapidly and individually and permits the quantitative analysis of microbial heterogeneity. It therefore offers many advantages over conventional measurements for both routine and more exploratory analyses of microbial properties. While the technique has been widely applied to the study of mammalian cells, is use in microbiology has until recently been much more limited, largely because of the smaller size of microbes and the consequently smaller optical signals obtainable from them. Since these technical barriers no longer hold, flow cytometry with appropriate stains has been used for the rapid discrimination and identification of microbial cells, for the rapid assessment of viability and of the heterogeneous distributions of a wealth of other more detailed physiological properties, for the analysis of antimicrobial drug-cell interactions, and for the isolation of high-yielding strains of biotechnological interest. Flow cytometric analyses provide an abundance of multivariate data, and special methods have been devised to exploit these. Ongoing advances mean that modern flow cytometers may now be used by nonspecialists to effect a renaissance in our understanding of microbial heterogeneity.

Survival of cells and DNA of Aeromonas salmonicida released into aquatic microcosms

The survival of the bacterial fish pathogen Aeromonas salmonicida, and persistence of its DNA, were monitored in aquatic microcosms using selective culture and most probable number PCR. Bacterial cells and naked DNA were released into natural non-sterile microcosms consisting of lake sediment overlayered with lake water. Two different types of surface sediment were used. One was sandy in character, taken from the shoreline whilst the other was a littoral loamy surface mud. Inoculated cells and naked DNA became undetectable from water overlayers within 4 weeks of release. Colony counts of Aer. salmonicida declined below detectable limits after 4 weeks in loamy sediment or 7 weeks in sandy sediment; however, naked DNA and DNA from released cells remained detectable for more than 13 weeks.

Survival and Activity of lux-Marked Aeromonas salmonicida in Seawater

The fish pathogen Aeromonas salmonicida was chromosomally marked with genes encoding bacterial luciferase, luxAB, isolated from Vibrio fischeri, resulting in constitutive luciferase production. During exponential growth in liquid batch culture, luminescence was directly proportional to biomass concentration, and luminometry provided a lower detection limit of approximately 10(sup3) cells ml(sup-1), 1 order of magnitude more sensitive than enzyme-linked immunosorbent assay detection. In sterile seawater at 4(deg)C, lux-marked A. salmonicida entered a dormant, nonculturable state and population activity decreased rapidly. The activity per viable cell, however, increased by day 4, indicating that a proportion of the population remained active and culturable. Putative dormant cells were not resuscitated after the addition of a range of substrates.

Dormant/unculturable cells of the fish pathogen Aeromonas salmonicida

Viable cells of Aeromonas salmonicida remained in experimental marine systems after plate counts indicated an absence of culturable cells. These so-called viable but nonculturable (VBNC) cells were coccoid and smaller than their normal culturable counterparts. There was no reduction in lipopolysaccharide of the VBNC cells. There was an alteration in protein composition, however, with a decline in some (15, 70, 30, 22, and 17 kDa), but an increase in another protein (49 kDa). A significant loss of DNA occurred. The VBNC cells responded to fluorescent antibodies prepared against A. salmonicida by developing enlarged and bizarre shapes in the presence of yeast extract and nalidixic acid (the direct viable count technique), and they demonstrated respiratory activity. It was concluded that A. salmonicida survived in seawater, but major morphological changes occurred with cells retaining some viability but losing pathogenicity to Atlantic salmon (Salmo salar).

Cell-surface properties of the food- and water-borne pathogen Aeromonas hydrophila when stored in buffered saline solutions

Aeromonas hydrophila, a ubiquitous inhabitant of aquatic environments, commonly expresses several cell-surface properties that may contribute to virulence. Since many aquatic microorganisms in hostile environments can withstand starvation conditions for long periods, we examined the effect of storage under nutrient-poor conditions on the expression of cell-surface properties of this pathogen. Phenotypes studied were: (1) cell-surface hydrophobicity and charge, and (2) the ability to bind connective-tissue proteins and lactoferrin. Our results suggest that the response of A. hydrophila to nutrient-poor conditions is regimen specific. Generally, A. hydrophila cells became more hydrophobic and significantly increased their ability to bind the iron-binding glycoprotein lactoferrin when the bacterium was stored under nutrient-poor conditions; however, under these conditions, the cells seemed to lose their ability to bind connective-tissue proteins.

Symbiotic Role of the Viable but Nonculturable State of Vibrio fischeri in Hawaiian Coastal Seawater

To achieve functional bioluminescence, the developing light organ of newly hatched juveniles of the Hawaiian squid Euprymna scolopes must become colonized by luminous, symbiosis-competent Vibrio fischeri present in the ambient seawater. This benign infection occurs rapidly in animals placed in seawater from the host's natural habitat. Therefore, it was surprising that colony hybridization studies with a V. fischeri-specific luxA gene probe indicated the presence of only about 2 CFU of V. fischeri per ml of this infective seawater. To examine this paradox, we estimated the total concentration of V. fischeri cells present in seawater from the host's habitat in two additional ways. In the first approach, the total bacterial assemblage in samples of seawater was collected on polycarbonate membrane filters and used as a source of both a crude cell lysate and purified DNA. These preparations were then assayed by quantitative DNA-DNA hybridization with the luxA gene probe. The results suggested the presence of between 200 and 400 cells of V. fischeri per ml of natural seawater, a concentration more than 100 times that revealed by colony hybridization. In the second approach, we amplified V. fischeri-specific luxA sequences from microliter volumes of natural seawater by PCR. Most-probable-number analyses of the frequency of positive PCR results from cell lysates in these small volumes gave an estimate of the concentration of V. fischeri luxA gene targets of between 130 and 1,680 copies per ml. From these measurements, we conclude that in their natural seawater environment, the majority of V. fischeri cells become nonculturable while remaining viable and symbiotically infective. Experimental studies indicated that V. fischeri cells suspended in natural Hawaiian seawater enter such a state within a few days.

Methionine uptake and cytopathogenicity of viable but nonculturable Shigella dysenteriae type 1

A pathogenic strain of Shigella dysenteriae type 1 was selected for study to elucidate the physiology and potential pathogenicity of organisms in the viable but nonculturable (VBNC) state in the environment. Studies in our laboratory have shown that S. dysenteriae type 1 survives in laboratory microcosms in the VBNC state for long periods of time, i.e., more than 6 months. VBNC cells of S. dysenteriae type 1 were found to retain cytopathogenicity for cultured HeLa cells. To determine whether VBNC S. dysenteriae type 1 expressed protein after loss of culturability, 35S-labelled methionine was added to suspensions of VBNC cells. Total cellular proteins were extracted and examined by autoradiography. Results indicate that VBNC S. dysenteriae type 1 is capable of both active uptake of methionine and incorporation of methionine into protein. Amino acid uptake and protein synthesis substantiate the viability of cells of S. dysenteriae type 1 in the VBNC state, i.e., although the cells are unable to be cultured on laboratory media by standard bacteriological methods, the cells remain metabolically active. Furthermore, VBNC cells of S. dysenteriae type 1 may pose a potential public health hazard that has not yet been recognized.