Section 1: Biological Hazards CHAPTER 1.1
Bacteria
1.1.1 AEROMONAS
Hazard Identification
What are Aeromonas?
Aeromonas species are gram-negative, non-spore-forming, bacteria, many of which are psychrotrophic (i.e. able to grow at low temperatures). Older references may state that these organisms are in the family Vibrionaceae, but they have recently been classified in a new family, the Aeromonadaceae, and this family now includes at least 14 described Aeromonas species.
Although a number of these species have been associated with human disease, the role of Aeromonas species as food-borne pathogens has yet to be confirmed. Aeromonas hydrophila, Aeromonas caviae, Aeromonas veronii biovar sobria and Aeromonas trota are the main species that are thought to cause gastrointestinal disease in man and it is considered that the main vehicle for these organisms is drinking water. Many Aeromonas species can be divided into two groups based on the temperature range at which strains are able to grow, and within a specific species some strains are psychrotophic, while others are mesophilic (not able to grow below 10 °C). For A. hydrophila, evidence suggests that those strains that are pathogenic to humans are mesophilic, whereas psychrotrophic strains are pathogenic to fish.
Occurrence in Foods
Aeromonas species are common contaminants in unprocessed foods and on occasions numbers can be high, exceeding 106 cfu gâ1. Because of their widespread occurrence it is thought likely that not all strains of Aeromonas species are pathogenic. Aeromonas species have been isolated from the following food commodities: fresh vegetables; salads; fish; seafood; raw meats including beef, lamb, pork and poultry; and raw milk as well as high-pH cheeses produced from raw milk. Aeromonas species have also, on occasions, been isolated from some processed foods including pasteurised milk, whipped cream, ice cream and ready-to-eat animal products.
Possible gastroenteritis-causing species have been isolated from most of the above food groups. However, A. caviae is more commonly isolated from vegetables and salad while A. hydrophilia is more commonly isolated from meat, fish and poultry.
Hazard Characterisation
Effects on Health
Although there is increasing evidence to suggest that A. hydrophila, A. caviae and A. veronii biovar sobria are causative agents of food-borne gastroenteritis in humans, this is still a subject of debate. However, aeromonads are often detected in gastrointestinal infections.
The infectious dose is unknown, although data suggests that it is probably high, probably >106 cells. Volunteer feeding studies involving ingesting high numbers of A. hydrophila cells (>107) have been inconclusive, whereas the organism has been isolated from the stools of divers who became ill after taking in small amounts of contaminated water. Gastroenteritis associated with Aeromonas species is most frequently reported in young children, although it can occur in individuals of any age with the number of cases peaking in the summer months.
It is thought that when ingested, these organisms can cause gastrointestinal disease in healthy individuals, chronic enterocolitis in the elderly and septicaemia in the immunocompromised. Symptoms are thought to start to occur within 24â48 hours of ingestion of cells. Infection can manifest itself in one of two distinct forms. The more common form is a cholera-like illness (watery diarrhoea accompanied by a mild fever), sometimes accompanied by vomiting in children less than two years old. The less common form is a dysentery-like illness (diarrhoea with blood and mucus in the stools). The disease is usually self-limiting, lasting 1â7 days. Occasionally however, the diarrhoea can last for several months, or even longer (12 months plus). Rare cases of haemolytic uremic syndrome, following infection with Aeromonas species, have been linked to verocytotoxin-producing aeromonads.
Incidence and Outbreaks
Most Aeromonas infections are thought to be caused by contaminated water and there are few reported outbreaks of Aeromonas-associated gastroenteritis where food is the suspected vehicle of infection. These few incidents are mostly associated with seafood products such as raw oysters and clams, sashimi, cooked prawns, shrimp cocktail and raw fermented fish. The literature suggests that other food groups such as edible land snails, egg salad and smorgasbord (comprising shrimp and various ready-to-eat meat products) have also been involved.
Sources
Aeromonas species are ubiquitous, although the main source of the organisms is generally accepted as water. The organisms are found in flowing and stagnant fresh water, in water supplies (including chlorinated water), sewage and in marine waters, particularly those that border with fresh water such as in estuaries. Aeromonas species are also often found in household environments such as drains and sinks, and can be isolated from soil.
Aeromonads are found in aquatic animals such as frogs, fish and leeches, in reptiles and in domestic animals such as pigs, sheep, poultry and cows. They can also be carried by humans without symptoms on occasions, although carriage rates are higher in tropical or developing regions.
Growth and Survival Characteristics
The growth temperature range for Aeromonas species is variable, but is reported to be between <5 °C and 45 °C. Within a particular species there can be psychrotrophic strains (capable of growth at chill temperatures) and mesophilic strains (cannot grow below 10 °C). Although the optimum temperature for growth is generally reported as 28 °C, this figure is likely to vary depending on strain. Although environmental strains may not grow at 37 °C, many clinical strains can grow at 5â7 °C. A. hydrophila is reported to grow from 1â42 °C, with an optimum temperature of 28 °C.
Aeromonads are reported to survive freezing temperatures and have been isolated from frozen foods after storage for approximately two years.
The optimum pH range for the growth of aeromonads is between 6.5 and 7.5. The organisms are tolerant of pH values of up to 10 and many strains will grow down to pH 5.5 or less (under otherwise ideal conditions), but this characteristic is uncommon at chill temperatures. Aeromonads are inactivated at pH values <4.5.
Many aeromonads will not grow at salt levels >4%, although there are reports of some strains growing at concentrations of 6%. Studies have shown that when foods are stored at chill temperatures, Aeromonas species are unlikely to grow when the salt levels are more than 3â3.5% and pH values are below 6.0.
Aeromonas species are facultative anaerobes (capable of growth with or without oxygen). At chill temperatures however, it has been reported that growth rate is either unaffected, or possibly reduced, when fish is modified atmosphere/vacuum packaged. Modified atmospheres containing high levels of oxygen (>70%) have been shown to retard the growth of A. caviae on ready-to-eat vegetables at refrigeration temperatures.
Aeromonas species are not notably resistant to preservatives or sanitisers.
It is thought that their presence in chlorinated water is the result of post-treatment contamination or inefficiencies in the chlorination process.
Thermal Resistance
Aeromonads are not heat-resistant organisms and are readily inactivated by pasteurisation or equivalent processes. Decimal reduction times (D-values) of 3.20â6.23 min at 48 °C in raw milk have been recorded.
Control Options
Processing
At present, research suggests that if some Aeromonas strains are indeed food-borne pathogens, it is foods containing high numbers of the organisms that pose the greatest health risk.
Measures to reduce the likelihood of high numbers occurring should include: using treated water supplies in food processing; keeping foods chilled; and the thorough, frequent cleaning of equipment used to process foods, especially those that are not later cooked by the consumer, e.g. salads and vegetables.
Aeromonas species are easily inactivated by pasteurisation, or equivalent processes used by the food industry. Preventing the recontamination of heat-processed products, particularly those with a high water activity and neutral pH that are to be stored chilled, should ensure that aeromonads are not a potential health risk in these foods. Measures to reduce the risk of recontamination include keeping raw and cooked foods separate and implementing good handling and packaging practices.
Product Use
Aeromonas species should be considered as possible pathogens and it has been suggested that very young children, the elderly and the immunocompromised should avoid foods that could be contaminated with high numbers of these organisms.
Legislation
There is no specific legislation in the EU or the USA on levels of Aeromonas species in foods.
Sources of Further Information
Published
Janda, J.M. and Abbott, S.L. The genus Aeromonas: taxonomy, pathogenicity, and infection. Clinical Microbiology Reviews, 2010, 23(1), 35â73.
Daskalov, H. The importance of Aeromonas hydrophila in food safety. Food Control, 2006, 17, 474â83.
Isonhood, J.H. and Drake, M. Aeromonas species in foods. Journal of Food Protection, 2002, 65(3), 575â82.
On the Web
Guidelines for drinking water quality. Addendum: Microbiological agent in drinking water, 2nd edn. Aeromonas â World Health Organization. (2002). http://www.who.int/water_sanitation_health/dwq/en/admicrob2.pdf
1.1.2 ARCOBACTER
Hazard Identification
What is Arcobacter?
Arcobacters are potentially pathogenic, gram-negative, non-spore-forming bacteria, often described as aerotolerant Campylobacter-like organisms. Arcobacters are closely related to Campylobacter, and species in both genera share some similar morphological and metabolic characteristics. Both genera belong to the family Campylobacteraceae, however Arcobacter species can be differentiated from Campylobacter species by their ability to grow in air as well as at lower temperatures. There are currently nine described Arcobacter species, but it is Arcobacter butzleri, and more rarely Arcobacter cryaerophilus, that have been implicated in cases of human illness. On two occasions however, Arcobacter skirrowi has been linked to human infection, including in an individual suffering from chronic diarrhoea.
It is thought that the consumption of food contaminated with Arcobacter species may play a role in the transmission of these pathogens, although this has not yet been conclusively demonstrated. The most significant source of the organisms is thought to be contaminated water sources, however the organisms are also considered possible zoonotic agents (i.e. direct transmission may occur from animals to humans).
Occurrence in Foods
Arcobacters are associated with foods of animal origin and have been detected in beef, poultry, pork and lamb, but are most frequently found in poultry and pork products. Chicken carcasses and poultry processing plants are often contaminated with Arcobacter species and the organisms have been isolated from retail chicken and turkey products. However, evidence suggests that eggs are not usually contaminated with these bacteria. They have also been found in raw milk and shellfish (clams and mussels). Arcobacters are not routinely examined for in foods, and so their prevalence in other food types is unknown.
Hazard Characterisation
Effects on Health
Arcobacter butzleri is the most common Arcobacter species implicated in human disease. Those most at risk from developing the symptoms associated with Arcobacter infection are very young children, although any age group is susceptible. Asymptomatic infections are reported to occur.
The infective dose and incubation time is unknown. Clinical symptoms include abdominal pain, nausea and acute watery diarrhoea, typically lasting 3â15 days, although this can persist or re-occur on occasions for up to two months. Occasionally, vomiting, fever and chills are reported. Extra-intestinal disease such as septicaemia has also been documented occasionally.
Incidence and Outbreaks
The incidence of Arcobacter enteritis is unknown, and outbreaks caused by Arcobacter species have rarely been reported. One reason for this may be because these organisms are not routinely ...