Influence of Temperature, NaCI, and pH on the Growth of Aeromonas Hydrophila

Enhanced production of N-acetyl-glucosaminidase by marine Aeromonas caviae CHZ306 in bioreactor

N-Acetyl-glucosaminidases (GlcNAcases) are exoenzymes found in a wide range of living organisms, which have gained great attention in the treatment of disorders related to diabetes, Alzheimer's, Tay-Sachs', and Sandhoff's diseases; the control of phytopathogens; and the synthesis of bioactive GlcNAc-containing products. Aiming at future industrial applications, in this study, GlcNAcase production by marine Aeromonas caviae CHZ306 was enhanced first in shake flasks in terms of medium composition and then in bench-scale stirred-tank bioreactor in terms of physicochemical conditions. Stoichiometric balance between the bioavailability of carbon and nitrogen in the formulated culture medium, as well as the use of additional carbon and nitrogen sources, played a central role in improving the bioprocess, considerably increasing the enzyme productivity. The optimal cultivation medium was composed of colloidal α-chitin, corn steep liquor, peptone A, and mineral salts, in a 5.2 C:N ratio. Optimization of pH, temperature, colloidal α-chitin concentration, and kLa conditions further increased GlcNAcase productivity. Under optimized conditions in bioreactor (i.e., 34 °C, pH 8 and kLa 55.2 h-1), GlcNAcase activity achieved 173.4 U.L-1 after 12 h of cultivation, and productivity no less than 14.45 U.L-1.h-1 corresponding to a 370-fold enhancement compared to basal conditions.

Climatic Alterations Influence Bacterial Growth, Biofilm Production and Antimicrobial Resistance Profiles in Aeromonas spp

Climate change is expected to create environmental disruptions that will impact a wide array of biota. Projections for freshwater ecosystems include severe alterations with gradients across geographical areas. Life traits in bacteria are modulated by environmental parameters, but there is still uncertainty regarding bacterial responses to changes caused by climatic alterations. In this study, we used a river water microcosm model to evaluate how Aeromonas spp., an important pathogenic and zoonotic genus ubiquitary in aquatic ecosystems, responds to environmental variations of temperature and pH as expected by future projections. Namely, we evaluated bacterial growth, biofilm production and antimicrobial resistance profiles of Aeromonas species in pure and mixed cultures. Biofilm production was significantly influenced by temperature and culture, while temperature and pH affected bacterial growth. Reversion of antimicrobial susceptibility status occurred in the majority of strains and tested antimicrobial compounds, with several combinations of temperature and pH contributing to this effect. Current results highlight the consequences that bacterial genus such as Aeromonas will experience with climatic alterations, specifically how their proliferation and virulence and phenotypic resistance expression will be modulated. Such information is fundamental to predict and prevent future outbreaks and deleterious effects that these bacterial species might have in human and animal populations.

The use of the so-called 'tubs' for transporting and storing fresh fishery products

On-land transport/storage of fresh fishery products (FFP) for up to 3 days in 'tubs' of three-layered poly-ethylene filled with freshwater and ice was compared to the currently authorised practice (fish boxes of high-density poly-ethylene filled with ice). The impact on the survival and growth of biological hazards in fish and the histamine production in fish species associated with a high amount of histidine was assessed. In different modelling scenarios, the FFP are stored on-board in freshwater or seawater/ice (in tubs) and once on-land they are 'handled' (i.e. sorted or gutted and/or filleted) and transferred to either tubs or boxes. The temperature of the FFP was assumed to be the most influential factor affecting relevant hazards. Under reasonably foreseeable 'abusive' scenarios and using a conservative modelling approach, the growth of the relevant hazards (i.e. Listeria monocytogenes, Aeromonas spp. and non-proteolytic Clostridium botulinum), is expected to be < 0.2 log10 units higher in tubs than in boxes after 3 days when the initial temperature of the fish is 0°C ('keeping' process). Starting at 7°C ('cooling-keeping' process), the expected difference in the growth potential is higher (< 1 log10 for A. hydrophila and < 0.5 log10 for the other two hazards) due to the poorer cooling capacity of water and ice (tub) compared with ice (box). The survival of relevant hazards is not or is negligibly impacted. Histamine formation due to growth of Morganella psychrotolerans under the 'keeping' or 'cooling-keeping' process can be up to 0.4 ppm and 1.5 ppm higher, respectively, in tubs as compared to boxes after 3 days, without reaching the legal limit of 100 ppm. The water uptake associated with the storage of the FFP in tubs (which may be up to 6%) does not make a relevant contribution to the differences in microbial growth potential compared to boxes.

Predictive Modeling for the Growth of Aeromonas hydrophila on Lettuce as a Function of Combined Storage Temperature and Relative Humidity

This study developed predictive growth models of Aeromonas hydrophila on lettuce as a function of combined storage temperature (15-35°C) and relative humidity (RH, 60-80%) using a polynomial equation. The primary model of specific growth rate, lag time, and maximum population density showed a good fit (R2 ≥ 0.95) with a Gompertz equation. A secondary model was obtained using a quadratic polynomial equation. The appropriateness of the secondary model was verified by mean square error (0.0001-0.8848), bias factor (Bf = 0.962-1.009), and accuracy factor (Af = 1.002-1.104). The newly developed secondary models for A. hydrophila could be incorporated into the tertiary modeling program to predict the growth of A. hydrophila as a function of combined temperature and RH. The developed model may be useful to predict potential A. hydrophila growth on lettuce, which is important for food safety purpose during the overall food chain of lettuce from farm to table. It could offer reliable and useful information of growth kinetics for the quantification of microbial risk assessment of A. hydrophila on lettuce.

The Significance of Mesophilic Aeromonas spp. in Minimally Processed Ready-to-Eat Seafood

Minimally processed and ready-to-eat (RTE) seafood products are gaining popularity because of their availability in retail stores and the consumers' perception of convenience. Products that are subjected to mild processing and products that do not require additional heating prior to consumption are eaten by an increasing proportion of the population, including people that are more susceptible to foodborne disease. Worldwide, seafood is an important source of foodborne outbreaks, but the exact burden is not known. The increased interest in seafood products for raw consumption introduces new food safety issues that must be addressed by all actors in the food chain. Bacteria belonging to genus Aeromonas are ubiquitous in marine environments, and Aeromonas spp. has held the title "emerging foodborne pathogen" for more than a decade. Given its high prevalence in seafood and in vegetables included in many RTE seafood meals, the significance of Aeromonas as a potential foodborne pathogen and a food spoilage organism increases. Some Aeromonas spp. can grow relatively uninhibited in food during refrigeration under a broad range of pH and NaCl concentrations, and in various packaging atmospheres. Strains of several Aeromonas species have shown spoilage potential by the production of spoilage associated metabolites in various seafood products, but the knowledge on spoilage in cold water fish species is scarce. The question about the significance of Aeromonas spp. in RTE seafood products is challenged by the limited knowledge on how to identify the truly virulent strains. The limited information on clinically relevant strains is partly due to few registered outbreaks, and to the disputed role as a true foodborne pathogen. However, it is likely that illness caused by Aeromonas might go on undetected due to unreported cases and a lack of adequate identification schemes. A rather confusing taxonomy and inadequate biochemical tests for species identification has led to a biased focus towards some Aeromonas species. Over the last ten years, several housekeeping genes has replaced the 16S rRNA gene as suitable genetic markers for phylogenetic analysis. The result is a more clear and robust taxonomy and updated knowledge on the currently circulating environmental strains. Nevertheless, more knowledge on which factors that contribute to virulence and how to control the potential pathogenic strains of Aeromonas in perishable RTE seafood products are needed.

Species Distribution and Prevalence of Putative Virulence Factors in Mesophilic Aeromonas spp. Isolated from Fresh Retail Sushi

Aeromonas spp. are ubiquitous bacteria that have received increasing attention as human pathogens because of their widespread occurrence in food, especially seafood and vegetables. The aim of this work was to assess the species identity and phylogenetic relationship of 118 Aeromonas strains isolated from fresh retail sushi from three producers, and to characterize the isolates with respect to genetic and phenotypic virulence factors. We also evaluate the potential hazard associated with their presence in ready-to-eat seafood not subjected to heat treatment. Mesophilic Aeromonas salmonicida was most prevalent (74%), followed by A. bestiarum (9%), A. dhakensis (5%), A. caviae (5%), A. media (4%), A. hydrophila (2%), and A. piscicola (1%). All isolates were considered potentially pathogenic due to the high prevalence of genes encoding hemolysin (hlyA) (99%), aerolysin (aerA) (98%), cytotoxic enterotoxin (act) (86%), heat-labile cytotonic enterotoxin (alt) (99%), and heat-stable cytotonic enterotoxin (ast) (31%). The shiga-like toxins 1 and 2 (stx-1 and stx-2) were not detected. Moreover, there was heterogeneity in toxin gene distribution among the isolates, and the combination of act/alt/hlyA/aerA was most commonly detected (63%). β-hemolysis was species-dependent and observed in 91% of the isolates. All A. media and A. caviae strains were non-hemolytic. For isolates belonging to this group, lack of hemolysis was possibly related to the absence of the act gene. Swimming motility, linked to adhesion and host invasion, occurred in 65% of the isolates. Partial sequencing of the gyrB gene demonstrated its suitability as a genetic marker for Aeromonas species identification and for assessment of the phylogenetic relationship between the isolates. The gyrB sequence divergence within a given species ranged from 1.3 to 2.9%. A. bestiarum, A. salmonicida, and A. piscicola were the most closely related species; their sequences differed by 2.7-3.4%. The average gyrB sequence similarity between all species was 93%, demonstrating its acceptable taxonomic resolution. The presence of multiple species of potential pathogenic Aeromonas in fresh retail sushi raises new food safety issues related to the increased consumption of ready-to-eat food composed of raw ingredients.

Increasing genomic diversity and evidence of constrained lifestyle evolution due to insertion sequences in Aeromonas salmonicida

BACKGROUND

Aeromonads make up a group of Gram-negative bacteria that includes human and fish pathogens. The Aeromonas salmonicida species has the peculiarity of including five known subspecies. However, few studies of the genomes of A. salmonicida subspecies have been reported to date.

RESULTS

We sequenced the genomes of additional A. salmonicida isolates, including three from India, using next-generation sequencing in order to gain a better understanding of the genomic and phylogenetic links between A. salmonicida subspecies. Their relative phylogenetic positions were confirmed by a core genome phylogeny based on 1645 gene sequences. The Indian isolates, which formed a sub-group together with A. salmonicida subsp. pectinolytica, were able to grow at either at 18 °C and 37 °C, unlike the A. salmonicida psychrophilic isolates that did not grow at 37 °C. Amino acid frequencies, GC content, tRNA composition, loss and gain of genes during evolution, pseudogenes as well as genes under positive selection and the mobilome were studied to explain this intraspecies dichotomy.

CONCLUSION

Insertion sequences appeared to be an important driving force that locked the psychrophilic strains into their particular lifestyle in order to conserve their genomic integrity. This observation, based on comparative genomics, is in agreement with previous results showing that insertion sequence mobility induced by heat in A. salmonicida subspecies causes genomic plasticity, resulting in a deleterious effect on the virulence of the bacterium. We provide a proof-of-concept that selfish DNAs play a major role in the evolution of bacterial species by modeling genomes.

Aeromonas species in foods

Aeromonas species have been recognized as potential or emerging foodborne pathogens for more than 20 years. Aeromonads are estuarine bacteria and are ubiquitous in fresh water, fish and shellfish, meats, and fresh vegetables. Actual sourced foodborne outbreaks are few, but epidemiological evidence suggests that the bacterium can cause self-limiting diarrhea, with children being the most susceptible population. Most aeromonads are psychrotrophic and can grow in foods during cold storage. Aeromonads are not resistant to food processing regimes and are readily killed by heat treatment. A host of virulence factors are present, but the exact role of each in human disease has not been fully elucidated.

Effects of temperature, medium composition, pH, salt and dissolved oxygen on haemolysin and cytotoxin production by Aeromonas hydrophila isolated from oyster

The effects of temperature, medium composition, pH, salt content and dissolved oxygen (DO) on the production of haemolysin and cytotoxin by one strain of Aeromonas hydrophila isolated from oyster were investigated. Four media were tested: brain heart infusion broth (BHIB), casamino acid-yeast extract broth (CAYEB), nutrient broth (NB), and trypticase soy broth (TSB). BHIB was the best for toxin production even though the growth rates for Aeromonas hydrophila in all of these media were quite similar. Aeromonas hydrophila could produce haemolysin and cytotoxin at 37, 28 and 5 degrees C; however, the toxins were produced faster and were more stable at 28 degrees C than at 37 degrees C. Although Aeromonas hydrophila itself is tolerant to 5% (w/v) salt in BHIB and a pH range of pH 5.5 to 10.0, the production of haemolysin and cytotoxin was apparently decreased in the presence of 1-5% (w/v) NaCl or when the pH of the medium was greater or less than 7.2. The DO values in the culture medium during the stationary growth phase also seemed to affect toxin production; greater quantities of toxins were produced when the DO values were higher.

Incidence and toxigenicity of Aeromonas hydrophila in seafood

Three selective media, Oxoid Aeromonas agar (OA), blood ampicillin agar (BA) and starch ampicillin agar (SA) were used to evaluate the presence of Aeromonas hydrophila in 66 samples of oyster, shrimp, fish and surimi products. Oyster had the highest incidence, with 50% positive, whilst no A. hydrophila was found in the surimi. Of the three selective media, BA displayed the highest recovery rate of A. hydrophila from seafood. Forty-eight isolates from this survey were tested for their capability to produce hemolysin and cytotoxin. Hemolysin was produced by 79.2% of the isolates and cytotoxin was produced by 91.7% of the isolates in brain heart infusion broth. One of the toxin-producing isolates from oyster, strain 8-169, was further tested for growth and toxin production in oyster, shrimp and fish at various temperatures. This particular isolate grew best and had highest toxin production in oyster. Hemolysin and cytotoxin were produced earlier at 28 degrees C than at 37 degrees C, and titers of hemolysin were also higher at 28 degrees C. At 5 degrees C, it was able to grow and produce hemolysin in oyster.

Enterotoxigenicity, hemolytic activity and antibiotic resistance of Aeromonas spp. isolated from freshwater prawn marketed in Dhaka, Bangladesh

Aeromonas spp. were isolated from gills, swimmerets, eggs, stomachs and ventral muscles of freshwater prawns (Macrobrachium malcolmsonii) available in the local fish market of Dhaka, Bangladesh. The density of Aeromonas spp. on these different body parts of the prawn samples ranged from 1.1 +/- 0.2 x 10(4) to 1.5 +/- 0.16 x 10(7) cfu per gram. The viable counts of aeromonads, fecal coliforms (FC) and Escherichia coli gradually increased in prawn samples when stored at 4 C. At -20 C, the viable counts gradually decreased and became zero on the 12th day of storage. The isolation of A. sobria (56%) was more frequent than that of A. hydrophila (31%) and A. caviae (13%). In the rabbit ileal loop (RIL) model, fluid accumulation induced by live cultures and cell-free culture filtrates of 11 strains ranged from 0.5 to 1.5 and 0.5 to 1.7 ml/cm of gut, respectively. Of 11 enterotoxigenic strains, 7 were A. sobria and 4 were A. hydrophila. Enterotoxigenicity correlated with hemolytic activity on blood agar. All enterotoxigenic strains were uniformly sensitive to chloramphenicol and gentamicin and resistant to novobiocin and vancomycin. Isolation of enterotoxigenic and antibiotic-resistant Aeromonas from these prawn samples indicates possible public health problems for their handlers as well for raw prawn consumers.

Heat and acid sensitivity of motile Aeromonas: a comparison with other food-poisoning bacteria

The present study was undertaken to compare the heat and acid sensitivity of aeromonads with those of other food-poisoning bacteria. It became obvious that aeromonads were more sensitive to heat than Escherichia coli O157:H7, Staphylococcus aureus, and Salmonella typhimurium. Aeromonads were killed in peptone water within 2 min at 55 degrees C, while the other bacteria survived heating at 55 degrees C for more than 15 min. Aeromonas cells were also less resistant to heat in hamburger steaks. These findings suggest that Aeromonas infection can easily be prevented by heat treatment, although correct handling of food is required to avoid recontamination since aeromonads are very common in various kinds of food. E. coli, S. aureus and S. typhimurium cells survived in buffer at pH 3.2 and in foods seasoned with vinegar. By contrast, Aeromonas cells were found to be highly sensitive to acid. However, the resistance of Aeromonas to acid may be sufficient to allow it to infect the gastrointestinal tract since Vibrio parahaemolyticus, which causes numerous outbreaks of food-poisoning every year in Japan, was susceptible to acid to the same extent as Aeromonas.

Influence of growth temperature on the production of extracellular virulence factors and pathogenicity of environmental and human strains of Aeromonas hydrophila

The biochemical properties, virulence for mice and trout, and the extracellular virulence factors at 28 degrees and 37 degrees C of 11 environmental and nine human strains of Aeromonas hydrophila were compared. All the environmental isolates and four of the human group were virulent for trout at 3 x 10(7) cfu, but only human strains were able to cause death or lesions in mice by the intramuscular route. Extracellular virulence factors such as haemolysins, cytotoxins and proteases were also investigated in supernatant fluids of cultures grown at 28 degrees C and 37 degrees C. The production of haemolysins, caseinases, elastases and growth yields of environmental strains decreased sharply during cultivation at 37 degrees C but cytotoxins were produced to the same extent, or slightly less, than at 28 degrees C. The human strains differed from the environmental strains in response to growth temperatures: protease activity decreased at 37 degrees C, although growth yield was not affected, but more haemolysins and cytotoxins were produced by the virulent strains at this temperature than at 28 degrees C. Sodium caseinate SDS-PAGE of culture supernatant fluids of selected human strains revealed that temperature selectively inhibited the production of certain proteases.

Behavior of Aeromonas species at refrigeration temperatures

The ability of many strains of Aeromonas hydrophila and A. sobria to produce several types of virulence factors has been documented. The presence of Aeromonas in drinking water, as well as in river and saline waters and on various finfish and shellfish taken from them, has caused some concern relative to the role this bacterium plays as a causative agent of human gastroenteritis. The fairly common occurrence of Aeromonas on red meats, poultry and fresh produce and its ability to grow at 4 degrees C gives rise to further concern over public health risks which may be associated with consumption of these foods. A brief overview of the behavior of Aeromonas species at refrigeration temperatures is presented.

Growth of and toxin production by Aeromonas hydrophila and Aeromonas sobria at low temperatures

The effects of different temperatures on the growth and toxin production of Aeromonas hydrophila and Aeromonas sobria were studied. The results showed that these Aeromonas species are not only able to grow at low temperatures (e.g. at 4 and 10 degrees C) but may also produce cytotoxin, hemolysin and enterotoxin under suitable growth conditions.

Experimental evidence for toxin production by Aeromonas hydrophila and Aeromonas sobria in a meat extract at low temperatures

The ability of enterotoxigenic strains of Aeromonas hydrophila and Aeromonas sobria to produce exotoxins (enterotoxin and haemolysin) in a meat extract at low temperatures (5 and 12 degrees C) was investigated. All three strains incubated at 12 degrees C were enterotoxigenic and haemolytic in the meat extract after 5 days. At 5 degrees C, five of the six strains tested were able to produce these exotoxins after 8 days incubation while one strain was neither enterotoxigenic nor haemolytic after 5, 8 and 11 days. The possible involvement of performed toxin(s) in Aeromonas gastroenteritis is also discussed.

Production of exotoxins by Aeromonas spp. at 5 degrees C

The ability of 60 strains of Aeromonas to produce enterotoxin and haemolysin after cultivation at 5 degrees C for 7-10 d was investigated. The strains were isolated from lamb meat, offal, carcasses and faeces, and had previously been tested for their ability to produce these exotoxins at 37 degrees C. The results showed that some strains of Aeromonas hydrophila and A. sobria were capable of producing enterotoxin and haemolysin at 5 degrees C, but none of the A. caviae strains tested produced these two factors. Of the 30 A. hydrophila strains investigated 25 and 27 were enterotoxigenic and haemolytic respectively. Likewise, of the 24 A. sobria strains investigated 16 and 18 were enterotoxigenic and haemolytic respectively. The results indicate that certain strains of Aeromonas species, in particular A. hydrophila and A. sobria, are of potential public health significance in meats stored at refrigeration temperature.

Growth of Aeromonas hydrophila on fresh vegetables stored under a controlled atmosphere

The effects of controlled-atmosphere storage (CAS) on the survival and growth of Aeromonas hydrophila on fresh asparagus, broccoli, and cauliflower were examined. Two lots of each vegetable were inoculated with A. hydrophila 1653 or K144. A third lot served as an uninoculated control. Following inoculation, vegetables were stored at 4 or 15 degrees C under a CAS system previously shown to extend the shelf life of each commodity or under ambient air. Populations of A. hydrophila were enumerated on the initial day of inoculation and at various intervals for 10 days (15 degrees C) or 21 days (4 degrees C) of storage. Direct plating of samples with selective media was used to enumerate A. hydrophila. The organism was detected on most lots of vegetables as they were received from a commercial produce supplier. Without exception, the CAS system lengthened the time vegetables were subjectively considered acceptable for consumption. However, CAS did not significantly affect populations of A. hydrophila which survived or grew on inoculated vegetables.

Characterization of the Aeromonas hydrophila group isolated from retail foods of animal origin

During a recent survey of retail fresh foods of animal origin (fish and seafood, raw milk, poultry, and red meats) for organisms of the Aeromonas hydrophila group, we isolated representative strains from the various foods. In this study, we sought to characterize these isolates for biochemical properties and virulence-associated factors and to compare the food isolates with clinical isolates. We identified all food and clinical isolates as A. hydrophila and found that all isolates were typical in their biochemical reactions. Examination of the isolates for various virulence-associated factors indicated that most food and clinical isolates were serum resistant, beta-hemolytic, cytotoxin positive (against Y1 adrenal cells), hemagglutinin positive, Congo red positive, elastase positive, and staphylolysin positive. Mouse 50% lethal doses were log10 8 to 9 CFU for most isolates. All isolates had biotypes identical to those of enterotoxin-positive strains. The public health significance of these organisms in foods is not known at present, although their widespread occurrence and ability to grow competitively in foods kept at 5 degrees C represents a potential hazard.

The growth of Aeromonas hydrophila K144 in ground pork at 5 degrees C

The influence of NaCl, pH, atmosphere, and background microflora on the growth and/or survival of Aeromonas hydrophila K144 was studied in ground pork held at 5 degrees C. In ground pork, A hydrophila was sensitive to pH values below 6.0 in the form of either a low starting pH in the pork itself or induced by lactic acid bacteria action on added glucose. Growth of the organism is inhibited by NaCl levels of 3% (w/w) (approx. 4% brine content). A hydrophila grew in vacuum-packaged ground pork; its growth was diminished by the presence of the naturally occurring meat microflora. Except for pH values below 6.0, conditions which inhibited growth permitted survival of the organisms for extended periods. Data indicate that the growth of A. hydrophila in ground pork can be controlled by factors such as NaCl, pH, and background microflora. In general, measures designed to control other foodborne pathogens appear adequate to limit A. hydrophila.