FACTORS AFFECTING GROWTH OR SURVIVAL OF AEROMONAS HYDROPHILA IN FOODS

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.

Growth of Aeromonas hydrophila in the whey cheeses Myzithra, Anthotyros, and Manouri during storage at 4 and 12 degrees C

The fresh whey cheeses Myzithra, Anthotyros, and Manouri were inoculated with Aeromonas hydrophila strain NTCC 8049 (type strain) or with an A. hydrophila strain isolated from food (food isolate) at levels of 3.0 to 5.0 x 10(2) CFU/g of cheese and stored at 4 or 12 degrees C. Duplicate samples of cheeses were tested for levels of A. hydrophila and pH after up to 29 days of storage. At 4 degrees C, A. hydrophila grew in Myzithra and Anthotyros with a generation time of ca. 19 h, but no growth was observed in Manouri. In Myzithra, average maximum populations of 8.87 log CFU/g (type strain) and 8.79 log CFU/g (food isolate) were recorded after 20 and 22 days of storage at 4 degrees C, respectively. The average maximum populations observed in Anthotyros stored at 4 degrees C were 6.72 log CFU/g (food isolate) and 6.13 log CFU/g (type strain) and were observed after 15 and 16 days of storage, respectively. A. hydrophila grew rapidly and reached high numbers in cheeses stored at 12 degrees C. The average generation times were 3.7 and 3.9 h (Myzithra), 4.1 and 6.1 h (Anthotyros), and 8.0 and 9.2 h (Manouri) for the type strain and the food isolate, respectively. Among the different whey cheese trials, the highest A. hydrophila population recorded (10.13 log CFU/g) was in Myzithra that had been inoculated with the food isolate after 8 days of storage at 12 degrees C. To prevent A. hydrophila growth in whey cheeses, efforts must be focused on preventing postprocessing contamination and temperature abuse during transportation and storage.

The effects of temperature, water activity and pH on the growth of Aeromonas hydrophila and on its subsequent survival in microcosm water

AIMS

The influence of temperature, water activity and pH on the growth of Aeromonas hydrophila, and on its survival after transfer in nutrient-poor water were assessed.

METHODS AND RESULTS

Experiments were carried out according to a Box-Behnken matrix at 10-30 degrees C, 0.95-0.99 water activity (aw) and pH 5-9. The effect of each factor on the kinetic parameters of growth (i.e. the maximal specific growth rate, mumax, and the lag time, lambda) and on the decline of the bacteria in microcosm water (time to obtain a reduction of 5 log, T5 log) were studied by applying central composite design.

CONCLUSIONS

The major effect of temperature and water activity on the growth of A. hydrophila was highlighted, whereas the effect of pH in these experimental conditions was not significant. Models describing the effect of environmental parameters on the growth of A. hydrophila were proposed. The effect of the growth environment, and particularly the incubation temperature, have an influence on the survival ability of the bacteria in nutrient-poor water.

SIGNIFICANCE AND IMPACT OF THE STUDY

The Box-Behnken design was well suited to determine the influence of environmental factors on the growth of A. hydrophila and to investigate the effect of previous growth conditions on its survival in microcosm water.

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.

Thermal and biological treatments to control psychrotrophic pathogens

Over the past decade, advances in egg processing technologies have permitted commercial production of ultrapasteurized liquid whole egg (LWE) products with a shelf-life of greater than 10 wk at 4 C. The inactivation and control of psychrotrophic pathogens such as Listeria monocytogenes and Aeromonas hydrophila in extended shelf-life LWE and conventionally pasteurized egg products is an ongoing food safety concern. This manuscript reports on the common features of these two psychrotrophic pathogens, their incidence in egg products, and their survival, growth potential, and heat resistance in liquid egg. Furthermore, this manuscript reports in detail on the results of two specific studies conducted in our laboratory whose objectives were: 1) to determine the heat resistance (D-values) of A. hydrophila in LWE using a low-volume immersed sealed glass capillary tube (ISCT) procedure; 2) to assess the impact of methodology (i.e., ISCT procedure vs a conventional capped test tube procedure) on the apparent thermal resistance of A. hydrophila; and 3) to report on the use of the bacteriocin nisin to restrict the survival of L. monocytogenes in ultrapasteurized LWE stored at refrigeration temperatures.

Behaviour of Yersinia enterocolitica and Aeromonas hydrophila in skim milk during fermentation by various lactobacilli

In this study, behaviour of Yersinia enterocolitica and Aeromonas hydrophila in skim milk during fermentation by various Lactobacillus sp. were determined. pH values of the skim milk samples were also examined during fermentation. The amount of produced lactic acid and diacetyl/acetoin productions of the Lactobacillus sp. were estimated. Antimicrobial effects of the lactobacilli on Y. enterocolitica and A. hydrophila were also determined by an agar diffusion method. While Y. enterocolitica was not inhibited and grew during fermentation, A. hydrophila was inhibited, in part, and the growth was retarded. Results were supported by the agar diffusion method for Y. enterocolitica, whereas inhibition activity was not found for A. hydrophila. The highest lactic acid productions were estimated in L. bulgaricus (7.50 mg/ml) and L. acidophilus (5.63 mg/ml) and four out of six Lactobacillus sp. were found to be diacetyl/acetoin producers.

Combined effect of gamma-irradiation and conventional cooking on Aeromonas hydrophila in meatball

Irradiation combined with a conventional cooking procedure was applied to meatball and the effects on bacterial load and inoculated Aeromonas hydrophila were determined. Meatball samples were irradiated by using a 60Co source at the dose levels of 0, 0.30, 0.75, 1.50, 2.50 kGy and cold stored at 4 +/- 1 degrees C for 7 days. Bacterial load and the count of A. hydrophila decreased when the irradiation dose level increased. A minimum inhibition effect was found at the dose of 0.30 kGy. Irradiation in combination with a conventional cooking procedure was found to be more effective in reducing A. hydrophila and the bacterial load in meatball. This study indicated that a dose of 0.75 kGy was sufficient to destroy approximately 10(4) cfu/g of A. hydrophila in meatball.

Emerging pathogens: Aeromonas spp

Aeromonas spp. are Gram-negative rods of the family Vibrionaceae. They are normal water inhabitants and are part of the regular flora of poiquilotherm and homeotherm animals. They can be isolated from many foodstuffs (green vegetables, raw milk, ice cream, meat and seafood). Mesophilic Aeromonas spp. have been classified following the AeroKey II system (Altwegg et al., 1990; Carnahan et al., 1991). The major human diseases caused by Aeromonas spp. can be classified in two major groups: septicemia (mainly by strains of A. veronii subsp. sobria and A. hydrophila), and gastroenteritis (any mesophilic Aeromonas spp. but principally A. hydrophila and A. veronii). Most epidemiological studies have shown Aeromonas spp. in stools to be more often associated with diarrhea than with the carrier state; an association with the consumption of untreated water was also conspicuous. Acute self-limited diarrhea is more frequent in young children, in older patients chronic enterocolitis may also be observed. Fever, vomiting, and fecal leukocytes or erythrocytes (colitis) may be present (Janda, 1991). The main putative virulence factors are: exotoxins, endotoxin (LPS), presence of S-layers, fimbriae or adhesins and the capacity to form capsules.

An example of the stages in the development of a predictive mathematical model for microbial growth: the effects of NaCl, pH and temperature on the growth of Aeromonas hydrophila

The stages involved in developing a predictive model are illustrated using data describing the effects of temperature (3-20 degrees C), NaCl concentration (0.5-4.5% w/v) and pH (4.6-7.0) on the aerobic growth of Aeromonas hydrophila (cocktail of 6 strains). Optical density measurements using micro-titre plates were used as an initial screen, to determine the appropriate sampling times for viable counts to be made and to determine the approximate boundaries for growth. Growth curves were generated from viable counts and fitted using a modified Gompertz equation. Quadratic response surface equations were fitted to the log of lag and generation times, in response to the variables of temperature, NaCl and pH (in terms of hydrogen ion concentration). The effects of various combinations of these controlling factors are described. Comparisons between predicted growth rates and lag times from our response surface equations and other models for growth of A. hydrophila, developed with viable count data and optical density measurements, are made, together with comparisons with data from the literature on the growth of this bacterium in foods.

The public health significance of Aeromonas spp. in foods

There is now evidence that some strains of Aeromonas species are enteropathogens. Such strains possess virulence properties, such as the ability to produce enterotoxins, cytotoxins, haemolysins and/or the ability to invade epithelial cells. Strains with these properties are common contaminants of drinking water and a wide range of foods. Contact or consumption of contaminated water, especially in summer, is a major risk factor in Aeromonas-associated gastroenteritis. Aeromonas-contaminated foods may also be vehicles of infection. Given the properties of strains that have been described in foods it has been suggested that food-borne illness could result not only from colonization and in vivo expression of virulence factors, but possibly also by intoxication following ingestion of foods that have been stored for a period of time, even under refrigeration. This paper reviews what is known about Aeromonas spp. in foods, their expression of virulence determinants, particularly at refrigeration temperatures, and the questions remaining to be answered to evaluate the risk they pose, so that an appropriate public health response can be determined.

Growth characteristics of motile Aeromonas spp. isolated from different environments

The growth of 80 strains of motile Aeromonas spp. derived from environments with temperatures above 25 degrees C and below 15 degrees C, respectively, were examined at five temperatures (5 degrees C, 10 degrees C, 25 degrees C, 37 degrees C and 44 degrees C) and four salt levels (0.05%, 2%, 4% and 6% NaCl). Sixty-one strains were further examined at two pH levels (pH 7.3 and pH 5.3). All strains grew at 25 degrees C and 10 degrees C with the majority of the isolates proliferating from approx. 10(2) to approx. 10(7) cfu/ml within 1 and 3 days, respectively. In contrast, there were significant differences in the proportion of isolates able to grow at 5 degrees C and 37 degrees C depending on the temperature of their source of isolation. The ecological background of the organisms thus influences their thermal growth range and their ability to proliferate at body temperature, a highly significant factor in infective disease. At 25 degrees C and pH 7.3, all strains grew in 0.05% NaCl, 96% grew in 2% NaCl, 96% grew in 2% NaCl while few grew in broth containing 4% or 6% NaCl. Lowering the pH to 5.3 with lactic acid caused a marked increase in the lag phase at 25 degrees C and prevented growth of a large number of isolates at suboptimal conditions. Thus, none of the isolates from warm environments and only 8% of the isolates from cold environments grew at this pH at 5 degrees C. The observed differences in growth optima between strains from different environments are discussed in relation to food- or waterborne infection.

Distribution and characteristics of Aeromonas in food and drinking water in Denmark

A total of 970 Danish commercial foods and drinking water samples were examined for the presence of motile Aeromonas spp. With a detection limit of 10(2)/g the frequent prevalence in raw foods was confirmed. Aeromonas occurred in 7% of 779 samples of prepared foods; most frequently in whipped cream from ice cream parlors (28%) and mayonnaise salads (10%) with numbers occasionally exceeding 10(5)/g. The prevalence in drinking water was 28% with a detection limit of 1/100 ml. A hydrophila was the dominating species in both food and water. Hemolysin production was demonstrated in 37% of the 51 isolates tested with 10% having high titers.

A note on Aeromonas spp. from chickens as possible food-borne pathogens

The possible role of Aeromonas spp. as potential food-borne psychrotrophic pathogens was investigated by examining organisms isolated from processed raw chicken for their biochemical characteristics, ability to produce exotoxins and to grow at chill temperatures. These strains, in particular A. sobria, with identical characteristics to human diarrhoea-associated aeromonads were readily found. Chicken, and human and environmental (water) strains characterized in a previous study, were investigated for their ability to grow at refrigeration temperatures (5 +/- 2 degrees C) and, for selected strains, the theoretical minimum temperature for growth (Tmin) was determined from the growth pattern in a temperature gradient incubator. All enterotoxigenic chicken strains tested were typical mesophiles, with an optimal growth temperature of approximately 37 degrees C and Tmin values approximately 4.5 degrees C. They were rapidly outgrown by a psychrotrophic Pseudomonas sp. typical of spoilage biota found on food. Enterotoxin was not produced below 15 degrees C by any of the toxigenic food strains tested. The Aeromonas strains isolated from chickens in this study seem unlikely therefore to be a significant health risk, provided the chickens are properly stored and cooked. This would appear to be substantiated by the lack of reports of food-associated outbreaks of illness from these sources.