Staphylococcus aureus – Food Standards

 

 

Staphylococcus aureus

Staphylococcus aureus is a bacterium that causes staphylococcal food poisoning, a form of
gastroenteritis with rapid onset of symptoms. S. aureus is commonly found in the
environment (soil, water and air) and is also found in the nose and on the skin of humans.

Description of the organism

S. aureus is a Gram-positive, non-spore forming spherical bacterium that belongs to the
Staphylococcus genus. The Staphylococcus genus is subdivided into 32 species and
subspecies. S. aureus produces staphylococcal enterotoxin (SE) and is responsible for
almost all staphylococcal food poisoning (Montville and Matthews 2008; FDA 2012).
S. intermedius, a Staphylococcus species which is commonly associated with dogs and
other animals, can also produce SE and has been rarely associated with staphylococcal
food poisoning (Talan et al. 1989; Khambaty et al. 1994; Le Loir et al. 2003).

Growth and survival characteristics

The growth and survival of S. aureus is dependent on a number of environmental factors
such as temperature, water activity (aw), pH, the presence of oxygen and composition of the
food (refer to Table 1). These physical growth parameters vary for different S. aureus strains
(Stewart 2003).

The temperature range for growth of S. aureus is 7–48°C, with an optimum of 37°C.
S. aureus is resistant to freezing and survives well in food stored below -20°C; however,
viability is reduced at temperatures of -10 to 0°C. S. aureus is readily killed during
pasteurisation or cooking. Growth of S. aureus occurs over the pH range of 4.0–10.0, with
an optimum of 6–7 (ICMSF 1996; Stewart 2003).

S. aureus is uniquely resistant to adverse conditions such as low aw, high salt content and
osmotic stress. In response to low aw, several compounds accumulate in the bacterial cell,
which lowers the intracellular aw to match the external aw (Montville and Matthews 2008). As
such, most S. aureus strains can grow over a aw range of 0.83 to >0.99 (FDA 2012).
S. aureus is a poor competitor, but its ability to grow under osmotic and pH stress means
that it is capable of thriving in a wide variety of foods, including cured meats that do not
support the growth of other foodborne pathogens (Montville and Matthews 2008).

S. aureus is a facultative anaerobe so can grow under both aerobic and anaerobic
conditions. However, growth occurs at a much slower rate under anaerobic conditions
(Stewart 2003).

For a non-sporing mesophilic bacterium, S. aureus has a relatively high heat resistance
(Stewart 2003). The observed average decimal reduction value (D-value, the value at which
the initial concentration of bacterial cells would be reduced by 1 log10 unit) was 4.8–6.6 min
at 60°C when heated in broth (Kennedy et al. 2005). The bacteria has a higher heat
resistance when it is encapsulated in oil, with a D-value at 60°C of 20.5 min for S. aureus in
fish and oil (Gaze 1985). An extremely heat resistant strain of S. aureus (D-value at 60°C of
>15 min in broth) has been recovered from a foodborne outbreak in India (Nema et al.
2007).

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Several chemical preservatives, including sorbates and benzoates, inhibit the growth of
S. aureus. The effectiveness of these preservatives increases as the pH is reduced. Methyl
and propyl parabens are also effective (Stewart 2003; Davidson and Taylor 2007).

Table 1: Limits for growth of S. aureus and enterotoxin production when other conditions are
near optimum (ICMSF 1996)

pH

Temperature (°C)

Bacterial Growth

Enterotoxin Production

Optimum

Range

Optimum

37

6–7

0.98

7–48

4–10

40–45

7–8

0.98

Range

10–48

4.5–9.6

Water activity

0.83–>0.99

0.87–>0.99

Symptoms of disease

Staphylococcal food poisoning symptoms generally have a rapid onset, appearing around
3 hours after ingestion (range 1–6 hours). Common symptoms include nausea, vomiting,
abdominal cramps and diarrhoea. Individuals may not demonstrate all the symptoms
associated with the illness. In severe cases, headache, muscle cramping and transient
changes in blood pressure and pulse rate may occur. Recovery is usually between 1–3 days
(Stewart 2003; FDA 2012). Fatalities are rare (0.03% for the general public) but are
occasionally reported in young children and the elderly (4.4% fatality rate) (Montville and
Matthews 2008).

S. aureus can cause various non-food related health issues such as skin inflammations
(e.g. boils and styes), mastitis, respiratory infections, wound sepsis and toxic shock
syndrome (Stewart 2003; Montville and Matthews 2008).

Virulence and infectivity

Staphylococcal food poisoning is an intoxication that is caused by the ingestion of food
containing pre-formed SE (Argudin et al. 2010). There are several different types of SE;
enterotoxin A is most commonly associated with staphylococcal food poisoning. Enterotoxins
D, E and H, and to a lesser extent B, G and I, have also been associated with
staphylococcal food poisoning (Seo and Bohach 2007; Pinchuk et al. 2010).

SEs are produced during the exponential phase of S. aureus growth, with the quantity being
strain dependent. Typically, doses of SE that cause illness result when at least
105 – 108 cfu/g of S. aureus are present (Seo and Bohach 2007; Montville and Matthews
2008). Most genes for SEs are located on mobile elements, such as plasmids or prophages.
As such, transfer between strains can occur, modifying the ability of S. aureus strains to
cause disease and contributing to pathogen evolution (Argudin et al. 2010; Pinchuk et al.
2010).

S. aureus produces SEs within the temperature range of 10–48°C, with an optimum of
40–45°C (refer to Table 1). As the temperature decreases, the level of SE production also
decreases. However, SEs remain stable under frozen storage. SEs are extremely resistant
to heating and can survive the process used to sterilise low acid canned foods. SE
production can occur in a pH range of 4.5–9.6, with an optimum of 7–8. Production of SE

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can occur in both anaerobic and aerobic environments; however, toxin production is
optimum in aerobic conditions (ICMSF 1996; Stewart 2003).

SEs are resistant to the heat and low pH conditions that easily destroy S. aureus bacteria.
The SEs are also resistant to proteolytic enzymes, hence SEs retain their activity in the
gastrointestinal tract after ingestion. SEs range in size from 22–28 kDa and contain a highly
flexible disulphide loop at the top of the N-terminal domain that is required for stable
conformation and is associated with the ability of the SE to induce vomiting (Argudin et al.
2010).

It has been suggested that SEs stimulate neuroreceptors in the intestinal tract which transmit
stimuli to the vomiting centre of the brain via the vagus nerve (Montville and Matthews 2008;
Argudin et al. 2010). In addition, SEs are able to penetrate the lining of the gut and stimulate
the host immune response. The release of inflammatory mediators, such as histamine,
causes vomiting. The host immune response also appears to be responsible for the damage
to the gastrointestinal tract associated with SE ingestion, with lesions occurring in the
stomach and upper part of the small intestine. Diarrhoea that can be associated with
staphylococcal food poisoning may be due to the inhibition of water and electrolyte re-
absorption in the small intestine (Argudin et al. 2010).

Mode of transmission

Staphylococcal food poisoning occurs when food is consumed that contains SE produced by
S. aureus. Food handlers carrying enterotoxin-producing S. aureus in their noses or on their
hands are regarded as the main source of food contamination via direct contact or through
respiratory secretions (Argudin et al. 2010).

Incidence of illness and outbreak data

Staphylococcal food poisoning is not a notifiable disease in Australia or New Zealand. There
were two reported outbreaks of staphylococcal food poisoning in Australia in 2011 and two
outbreaks reported in 2010. In New Zealand there were no outbreaks of staphylococcal food
poisoning in 2011 and two outbreaks reported in 2010 (Lim et al. 2012; OzFoodNet 2012a;
OzFoodNet 2012b). It is generally recognised that there may be significant under reporting
of staphylococcal food poisoning due to the short duration of illness and self-limiting
symptoms. In Australia it is estimated that S. aureus accounts for 1% of foodborne illness
caused by known pathogens (Hall et al. 2005).

In the European Union there were 0.07 reported cases of staphylococcal food poisoning per
100,000 population in 2011 (ranging from <0.01–0.45 per 100,000 population between countries). This was similar to the 2010 rate of 0.06 cases per 100,000 population (EFSA 2012; EFSA 2013). In the United States (US) the notification rate for vancomycin-intermediate S. aureus was 0.04 cases per 100,000 population in 2010, which was as increase from the 2009 rate of 0.03 (CDC 2012). It is estimated that in the US, S. aureus accounts for 2.6% of foodborne illness caused by 31 major pathogens (Scallan et al. 2011). The incidence of staphylococcal food poisoning is seasonal. Most cases occur in the late summer when temperatures are warm and food is stored improperly (Montville and Matthews 2008). 3 Foods associated with outbreaks of staphylococcal food poisoning include meat and meat products, poultry and egg products, milk and dairy products, salads, cream-filled bakery products and sandwich fillings. Foods that require extensive handling during preparation and are kept above refrigeration temperature (4°C) for extended periods after preparation are often involved in staphylococcal food poisoning (Argudin et al. 2010; FDA 2012) (refer to Table 2). Foods high in starch and protein are believed to favour SE production (Stewart 2003). Table 2: Selected major outbreaks associated with S. aureus (>50 cases and/or ≥1 fatality)

Food

Country

Comments

References

Year

2007

No. of
cases
(fatalities)
400 (1)

UHT milk

Paraguay Production line

(Weiler et al.
2011)

2006

113

Chicken and
rice

Austria

2000

13,420

Powdered
skim milk

Japan

(Schmid et al.
2007)

(Asao et al.
2003)

1998

4000 (16) Chicken,

Brazil

(Do Carmo et al.
2004)

roast beef,
rice and
beans

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operator
identified as
source of post-
pasteurisation
contamination
Kitchen worker
positive for
S. aureus
Production of
powdered skim
milk stopped
midway and
delayed for 9
hours due to
power cut, this
permitted
S. aureus
growth and SE
production. SE
survived the
pasteurisation
process that
destroyed
S. aureus
bacteria
Food
preparation
began over 48
hours before
food served,
food left at room
temperature for
one day. All
food handlers
positive for
S. aureus

Food handler
positive for
S. aureus
removed ham
casings without
gloves.
Improper
refrigeration,
prolonged
handling and
inadequate
reheating of
ham
Mushrooms
picked and
sorted by hand.
Mushrooms
stored without
refrigeration in
plastic bags,
this permitted
S. aureus
growth and SE
production.
Deficiencies in
sanitation.
Imported
product
Milk stored for
several hours in
inadequately
cooled tank
prior to
pasteurisation,
conditions
permitted
S. aureus
growth and SE
production

Year

1990

No. of
cases
(fatalities)
100

Food

Country

Comments

References

Ham

US

(Richards et al.
1993)

1989

99

Canned
mushrooms

US

(Levine et al.
1996)

Mid-1980s

>850

Chocolate
milk

US

(Evenson et al.
1988)

Occurrence in food

Despite S. aureus colonising a wide range of animals, people are the main reservoir of food
contamination (Montville and Matthews 2008). Prevalence of enterotoxigenic S. aureus in
food handlers is variable between industries and countries. Prevalence estimates from
several small studies range from 2% of food handlers in Italy (n=545) (Talarico et al. 1997),
12% of flight-catering staff in Finland (n=136) (Hatakka et al. 2000), 19% of restaurant
workers in Chile (n=102) (Figueroa et al. 2002) to 62% of fish processing factory workers in
India (n=87) (Simon and Sanjeev 2007).

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The udders and teats of cows are known sources of enterotoxigenic S. aureus, and the
occurrence of S. aureus in unpasteurised milk and cheese is common. The tonsils and skin
of pigs, chickens and turkeys often harbour S. aureus, and are also potential sources of
S. aureus contamination (Stewart 2003).

A survey of food from retail markets and dairy farms in Turkey was performed between 2007
and 2008. Enterotoxigenic S. aureus was found in 11.3% of meat (n=115), 10.2% of
unpasteurised milk (n=303), 8.0% of dairy products (n=452), 3.5% of bakery products
(n=141) and 2.3% or ready-to-eat products (n=44) (Aydin et al. 2011).

An Italian survey performed between 2003 and 2005 indicated that 9.2% of dairy products
(n=641) and 5.0% of meat products (n=993) were positive for enterotoxigenic S. aureus
(Normanno et al. 2007). In Japan a retail survey performed between 2002 and 2003 found
17.6% of raw chicken meat (n=444) were positive for enterotoxigenic S. aureus (Kitai et al.
2005).

Host factors that influence disease

All people are believed to be susceptible to staphylococcal food poisoning. However, the
severity of symptoms may vary depending on the amount of SE consumed in the food and
the general health of individuals. The young and elderly are more likely to develop more
serious symptoms (FDA 2012).

Dose response

A human feeding trial in the 1960s demonstrated that 20–25 µg (0.4 µg/kg body weight) of
enterotoxin B caused illness (Raj and Bergdoll 1969). However, more recently it has been
reported that less than 1.0 µg of SE is sufficient to cause staphylococcal food poisoning
(FDA 2012). In fact evidence from outbreaks indicates that ingestion of less than 200 ng of
enterotoxin A is sufficient to cause illness in susceptible individuals (Evenson et al. 1988;
Asao et al. 2003).

Recommended reading and useful links

FDA (2012) Bad bug book: Foodborne pathogenic microorganisms and natural toxins
handbook, 2nd ed, US Food and Drug Administration, Silver Spring, p. 87–92.
http://www.fda.gov/Food/FoodborneIllnessContaminants/CausesOfIllnessBadBugBook/ucm2
006773.htm

Pinchuk IV, Beswick EJ, Reyes VE (2010) Staphylococcal enterotoxins. Toxins 2:2177–2197

Stewart CM (2003) Staphylococcus aureus and staphylococcal enterotoxins. Ch 12 In:
Hocking AD (ed) Foodborne microorganisms of public health significance. 6th ed, Australian
Institute of Food Science and Technology (NSW Branch), Sydney, p. 359–379

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References

Argudin MA, Mendoza MC, Rodicio MR (2010) Food poisoning and Staphylococcus aureus
enterotoxins. Toxins 2(7):1751–1773

Asao T, Kumeda Y, Kawai T, Shibata T, Oda H, Haruki K, Nakazawa H, Kozaki S (2003) An
extensive outbreak of staphylococcal food poisoning due to low-fat milk in Japan: Estimation
of enterotoxin A in the incriminated milk and powdered skim milk. Epidemiology and Infection
130:33–40

Aydin A, Sudagidan M, Muratoglu K (2011) Prevalence of staphylococcal enterotoxins, toxin
genes and genetic relatedness of foodborne Staphylococcus aureus strains isolated in the
Marmara region of Turkey. International Journal of Food Microbiology 148:99–106

CDC (2012) Summary of notifiable diseases – United States, 2010. Morbidity and Mortality
Weekly Report 59(53):1–111

Davidson PM, Taylor TM (2007) Chemical preservatives and natural antimicrobial
compounds. Ch 33 In: Doyle MP, Beuchat LR (eds) Food microbiology: Fundamentals and
frontiers. 3rd ed, ASM Press, Washington D.C., p. 713–745

Do Carmo LS, Cummings C, Linardi VR, Dias RS, De Souza JM, De Sena MJ,
Dos Santos DA, Shupp JW, Pereira RKP, Jett M (2004) A case study of a massive
staphylococcal food poisoning incident. Foodborne Pathogens and Disease 1(4):241–246

EFSA (2012) The European Union summary report on trends and sources of zoonoses,
zoonotic agents and foodborne outbreaks in 2010. EFSA Journal 10(3):2597

EFSA (2013) The European Union summary report on trends and sources of zoonoses,
zoonotic agents and foodborne outbreaks in 2011. EFSA Journal 11(4):3129

Evenson ML, Hinds MW, Berstein RS, Bergdoll MS (1988) Estimation of human dose of
staphylococcal enterotoxin A from a large outbreak of staphylococcal food poisoning
involving chocolate milk. International Journal of Food Microbiology 7:311–316

FDA (2012) Bad bug book: Foodborne pathogenic microorganisms and natural toxins
handbook, 2nd ed. US Food and Drug Administration, Silver Spring, p. 87–92.
http://www.fda.gov/Food/FoodborneIllnessContaminants/CausesOfIllnessBadBugBook/ucm2
006773.htm. Accessed 27 March 2013

Figueroa G, Navarrete P, Caro M, Troncoso M, Faundez G (2002) Carriage of
enterotoxigenic Staphylococcus aureus in food handlers. Revista Medica De Chile
130(8):859–864

Gaze JE (1985) The effect of oil on the heat resistence of Staphylococcus aureus. Food
Microbiology 2:277–283

Hall G, Kirk MD, Becker N, Gregory JE, Unicomb L, Millard G, Stafford R, Lalor K (2005)
Estimating foodborne gastroenteritis, Australia. Emerging Infectious Diseases 11(8):1257–
1264

Hatakka M, Bjorkroth KJ, Asplund K, Maki-Petays N, Korkeala HJ (2000) Genotypes and
enterotoxicity of Staphylococcus aureus isolated from the hands and nasal cavities of flight-
catering employees. Journal of Food Protection 63(11):1487–1491

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ICMSF (1996) Staphylococcus aureus. Ch 17 In: Microorganisms in food 5: Microbiological
specifications of food pathogens. Blackie Academic and Professional, London, p. 299–333

Kennedy J, Blair IS, McDowell DA, Bolton DJ (2005) An investigation of the thermal
inactivation of Staphylococcus aureus and the potential for increased thermotolerance as a
result of chilled storage. Journal of Applied Bacteriology 99:1229–1235

Khambaty FM, Bennett RW, Shah DB (1994) Application of pulsed-field gel electrophoresis
to the epidemiological characterization of Staphylococcus intermedius implicated in a food-
related outbreak. Epidemiology and Infection 113:75–81

Kitai S, Shimizu A, Kawano J, Sato E, Nakano C, Kitagawa H, Fujio K, Matsumura K,
Yasuda R, Inamoto T (2005) Prevalence and characterization of Staphylococcus aureus and
enterotoxigenic Staphylococcus aureus in retail raw chicken meat throughout Japan. The
Journal of Veterinary Medical Science 67(3):269–274

Le Loir Y, Baron F, Gautier M (2003) Staphylococcus aureus and food poisoning. Genetics
and Molecular Research 2(1):63–76

Levine WC, Bennett RW, Choi Y, Henning KJ, Rager JR, Hendricks KA, Hopkins DP,
Gunn RA, Griffin PM (1996) Staphylococcal food poisoning caused by imported canned
mushrooms. Journal of Infectious Diseases 173:1263–1267

Lim E, Lopez L, Borman A, Cressey P, Pirie R (2012) Annual report concerning foodborne
disease in New Zealand 2011. Ministry for Primary Industry, New Zealand.
http://www.foodsafety.govt.nz/science-risk/human-health-surveillance/foodborne-disease-
annual-reports.htm. Accessed 11 April 2013

Montville TJ, Matthews KR (2008) Food microbiology: An introduction. 2nd ed, ASM Press,
Washington D.C.

Nema V, Agrawal R, Kamboj DV, Goel AK, Singh L (2007) Isolation and characterization of
heat resistant entertoxigenic Staphylococcus aureus from a food poisoning outbreak in
Indian subcontinent. International Journal of Food Microbiology 117:29–35

Normanno G, La Salandra G, Dambrosio A, Quaglia NC, Corrente M, Parisi A, Santagada
G, Firinu A, Crisetti E, Celano GV (2007) Occurrence, characterization and antimicrobial
resistance of enterotoxigenic Staphylococcus aureus isolated from meat and dairy products.
International Journal of Food Microbiology 115:290–296

OzFoodNet (2012a) Monitoring the incidence and causes of diseases potentially transmitted
by food in Australia: Annual report of the OzFoodNet Network, 2010. Communicable
Diseases Intelligence 36(3):E213–E241

OzFoodNet (2012b) OzFoodNet Quarterly report, 1 July to 30 September 2011.
Communicable Diseases Intelligence 36(2):E188–E195

Pinchuk IV, Beswick EJ, Reyes VE (2010) Staphylococcal enterotoxins. Toxins 2:2177–2197

Raj HD, Bergdoll MS (1969) Effect of enterotoxin B on human volunteers. Journal of
Bacteriology 98(2):833–834

Richards MS, Rittman M, Gilbert TT, Opal SM, DeBuono BA, Neill RJ, Gemski P (1993)
Investigation of a staphylococcal food poisoning outbreak in a centralized school lunch
program. Public Health Reports 108(6):765–771

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