Food microbiology

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Food microbiology

• Chap. 14 Staphylococcus aureus

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Historical aspects and general considerations

• The link between staphylococci and foodborne
  illness was made 1914 when researchers found that
  drinking contaminated milk caused vomiting and
  diarrhea
• In 1930, G. M. Dack and coworkers voluntarily
  consumed fluids from cultures of a yellow
  hemolytics Staphylococcus isolated from
  contaminated cake

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Nomenclature, characteristics, and distribution
of staphylococcal-enterotoxin-producing
staphylococci

• The term staphylococci informally describes a
  group of small, spherical, gram-positive bacteria
• They are catalase positive, have typical
  gram-positive cell walls cantaining peptidoglycan
  and teichoic acids
• The genus Staphylococcus is subdivided into gt23
  species and subspecies

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Nomenclature, characteristics, and distribution
of staphylococcal-enterotoxin-producing
staphylococci

• Several species of Staphylococcus, including both
  coagulase-negative and coagulase-positive
  isolates, can produce staphylococcal enterotoxins
• Although several species can cause
  gastroenteritis, nearly all staphylococcal food
  poisoning is attributed to S. aureus

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Introduction and nomenclature of the
staphlyococcal enterotoxins

• The current classification scheme is based on
  antigenicity
• They used purified toxins to show that protective
  antibodies were made in animals
• Staphylococcal enterotoxins are named by letter
  in the order of their discovery
• Staphylococcal enterotoxins A, B, C, D, and E are
  the major types

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Introduction and nomenclature of the
staphlyococcal enterotoxins

• Staphylococcal enterotoxins G, H, J, and I are
  more recent discoveries
• An exotoxin produced by the S. aureus strain
  associated with toxic shock syndrome was
  initially called staphylococcal enterotoxin F

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Sources of S. aureusSources of staphylococcal
food contamination

• People are the main reservoir of S. aureus
• Human are natural carries and spread
  staphylococci to other people and to food
• In human, the interior of the nose is the main
  colonization site S. aureus also occurs on the
  skin
• S. aureus spreads by direct contact, through skin
  fragments, or through respiratory droplets
  produced when people cough or sneeze

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Sources of S. aureusSources of staphylococcal
food contamination

• Most staphylococcal food poisoning is traced to
  food contaminated by humans during preparation
• In addition to contamination by food handlers,
  meat grinders, knives, storage containers,
  cutting blocks, and saw blades may also introduce
  S. aureus into food

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Sources of S. aureusSources of staphylococcal
food contamination

• Conditions often associated with outbreaks of
  staphylococcal illness are
• inadequate refrigeration
• preparing foods too far in advance
• poor personal hygiene
• inadequate cooking or heating of food
• prolonged use of warming plates when serving
  foods

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Sources of S. aureusSources of staphylococcal
food contamination

• Animals are also S. aureus sources, for example,
  bovine mastitis is a serious problem for the
  dairy industry
• Mastitis is also a public health concern because
  the bacteria can contaminate milk and dairy
  products
• S. aureus is present in many food

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Sources of S. aureusSources of staphylococcal
food contamination

• The staphylococcus levels are usually low
  initially, lt100 colony-forming units (CFU)/g
• It can grow to high levels, gt106 CFU/g, and cause
  staphylococcal food poisoning under favorable
  conditions

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Resistance to adverse environmental conditions

• S. aureus is one of the most resistant
  non-spore-forming pathogen
• It can survive in a dry state and is isolated
  from air, dust, sewage, and water relatively
  easily
• S. aureus grows in medium containing 20 NaCl
  and survives at water activities of lt0.86

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Resistance to adverse environmental conditions

• Staphylococci have an efficient osmoprotectant
  system, several compounds accumulate in the cell
  or enhance its growth under osmotic stress
• Glycine betaine is the most important
  osmoprotectant
• Under extreme conditions of growth, the organism
  may not produce toxin

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Foodborne outbreaksincidence of Staphylococcal
food poisoning

• There is little reason to report staphylococcal
  food poisoning, because people usually recover in
  24 to 48 h and have no reason to go to the doctor
• It is the third most common confirmed bacteria
  foodborne illness the second in Taiwan (2005)

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Characteristics of disease

• Staphylococcal food poisoning is a self-limiting
  illness causing emesis (vomiting) after an
  unusually short time before onset (as little as
  30 min after eating)
• Vomiting, nausea, cramps, diarrhea, headaches,
  and/or prostration are common symptoms
• In all cases of diarrhea, vomiting was also
  present

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Characteristics of disease

• Symptoms usually develop within 6 h after eating,
  the mean incubation period is 4.4 h, although
  illness can appear in 30 min
• Death due to staphylococcal food poisoning is
  rare
• Treatment is usually minimal, although fluids are
  given when diarrhea and vomiting are severe

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Infective dose and susceptible populationsNumbers
of Staphylococci required

• According to the Food and Drug Administration,
  when levels of S. aureus are gt105 cell/g of food,
  there may be enough enterotoxin to cause illness
• Other studies suggest that 105 to 108 cells is
  the typical range, although smaller cell
  populations are sometimes implicated

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Toxin dose required

• The best data on the minimum toxic dose come from
  analysis of food recovered from outbreaks
• Many outbreaks are caused by 1 to 5 mg of
  ingested toxin per person
• A toxin dose of less than 1.0 microgram in
  contaminated food will produce symptoms of
  staphylococcal intoxication. This toxin level is
  reached when S. aureus populations exceed 100,000
  per gram (FDA)

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Toxin dose required

• Many factors contribute to the likelihood of
  getting staphylococcal food poisoning and its
  severity they include individual susceptibility
  to the toxin, how much food was eaten, and a
  persons overall health
• The toxin type may also be important

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Virulence factors and mechanisms of
pathogenicity Staphylococcal enterotoxin
structure-function associations

• Basic structural and biophysical features
• Staphylococcal enterotoxins are single 25- to
  28-kDa polypeptides
• Staphylococcal enterotoxins are very stable,
  their toxicity and antigenicity are not
  completely destroyed by boiling or even canning

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Virulence factors and mechanisms of
pathogenicity Staphylococcal enterotoxin
structure-function associations

• The temperatures needed to inactivate
  staphylococcal toxins are higher than those
  needed to kill S. aureues cells
• In many cases of staphylococcal food poisoning,
  no live bacteria are found in the food
• The cells are heat sensitive, but the toxins are
  heat resistant

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Enterotoxins and exotoxins

• An enterotoxin is a protein toxin released by a
  micro-organism
• Enterotoxin are very stable
• Both gram negative and gram positive bacteria
  produce exotoxins
• Most exotoxins can be destroyed by heating