Common Foodborne Pathogens - III

 Staphylococcus aureus Risk Profile 

Staphylococcal species can be described as small, gram-positive, non-motile, facultatively anaerobic, catalase-positive, spherical bacteria (cocci), which can be seen as pairs, short chains, or bunched in grape-like clusters under the microscope. Staphylococci are impossible to eradicate from the environment because many of the 32 species and their subspecies are potentially found in foods due to environmental, human, and animal contamination. Several species of genus staphylococcus (both coagulase-negative and positive strains) can produce highly heat-stable enterotoxins that cause gastroenteritis in humans.

 

S. aureus is a predominant human pathogen that is capable of causing staphylococcal food poisoning, toxic shock syndrome, pneumonia, postoperative wound infection, and nosocomial bacteremia. Many of the various extracellular excrete of S. aureus are act as virulence factors. Nevertheless, Staphylococcal enterotoxins can act as super-antigens capable of stimulating an elevated percentage of T-cells. 

 

Genus Staphylococci are mesophilic organisms, and their growth ranges from 7°C to 47.8°C, with 35°C being the optimum temperature for growth. The pH range between 4.5 and 9.3, with an optimum the growth recorded between 7.0 and 7.5. Besides, Staphylococci are atypical, which means that colonies can grow at low levels of water activity, with growth demonstrated at aW as low as 0.83 under ideal conditions, and the optimum growth occurs at aW of >0.99. Furthermore, S. aureus strains are highly tolerant of salts and sugars. Hence, S. aureus is one of the most resistant non-spore-forming human pathogens, which can survive for extended periods in a dry state. 

 

Staphylococcal Enterotoxins (SE)

Enterotoxins produced by the genus Staphylococci are single-chain proteins with molecular weights of 26,000 to 29,000, which are resistant to proteolytic enzymes, such as trypsin and pepsin that allows them to transit intact through the digestive tract. Five serotypes are responsible for the production of classical enterotoxin, which are SEA, SEB, SEC1,2,3, SED, and SEE, and the more recently described SEG, SEH, and SEI; all exhibit emetic activity. There are also SE-like enterotoxin serotypes SElJ-SElU, however, these SE-like designations have not been confirmed to exhibit emetic activity. Besides, different SE serotypes are similar in composition and biological activity, but are different in antigenicity and are identified serologically as separate proteins.

 

Sources

Staphylococci can be found in the air, dust, sewage, water, milk, and food, or on food equipment, environmental surfaces, humans, animals and widely dispersed in the environment. Staphylococcus food poisoning is frequently associated in meat and meat products, poultry and egg products, salads such as egg, tuna, chicken, potato, and macaroni; bakery products, such as cream-filled pastries, cream pies, and chocolate éclairs; sandwich fillings; and milk and dairy products. Staphylococcus are such a common existence that they are expected to exist in any or all foods that are handled directly by humans or are of animal origin unless heat processes are applied. Hence, Staphylococcus food poisoning frequently involved in foods that require considerable handling during preparation and are kept slightly above proper refrigeration temperatures for an extended period after preparation. Thus, avoiding time temperature abuse of food products that are at high risk is essential in preventing the proliferation of the Staphylococcus as well as subsequent production of enterotoxin.

 

The enterotoxin damage to humans is permanent once produced, because destruction of viable cells by heat does not destroy the biological activity of preformed staphylococcal enterotoxins, which are highly heat stable and can remain biologically active. Besides, Staphylococcus are present in 50% or more of healthy individuals in the society, where their nasal passages, throats, on the hair and skin are popular hosting sites. Nonetheless, Staphylococcus presence is even higher among those who associate with sick people and hospital environments. Contamination may occur due to direct contact from workers with hand or arm lesions caused by S. aureus, or coughing and sneezing, which is common during respiratory infections, where food handlers are frequently the source of food contamination.  However, equipment, environment and food contact surfaces also can be sources.

 

Disease

Staphylococcal food poisoning which is also called as staphyloenterotoxicosis or staphyloenterotoxemia is the condition caused by the enterotoxins, where treatment normally involves managing the complications.

 

Mortality:

Staphylococcus food poisoning is not usually lethal among heathy population, but it has been lethal for immunocompromised or the elderly, infants, and severely debilitated people.

 

Infective dose:

The intoxication dose of SE is less than 1.0 microgram, where such toxin levels are reached when S. aureus populations exceed 100,000 organisms/g in food. Hence, it is an indication of unsanitary conditions of the food being considered and the product can be rendered injurious to human health. Besides, ingestion of 100 to 200 ng of enterotoxin can cause symptoms in highly sensitive people. Nonetheless, S. aureus population may be significantly different at the time of analysis, and not representation of the highest population occurred in the product, which should be taken into consideration when examining foods.

 

Onset:

Usually 1 to 7 hours.

The symptoms are usually rapid and acute in many cases, which depends on individual’s susceptibility to the toxin, amount of toxin ingested, and general health.

 

Complications:

The most common complication is dehydration caused by diarrhea and vomiting, whereas Staphylococcus food poisoning generally causes self-limiting, acutely intense infection in most people and not everyone demonstrate all symptoms associated with the illness. 

Symptoms:

Once ingested a contaminated food product, the enterotoxin may rapidly produce symptoms, which commonly include nausea, abdominal cramping, vomiting, and diarrhea. In addition, dehydration, headache, muscle cramping, and transient changes in blood pressure and pulse rate may occur in more severe cases.

 

Duration:

The illness is relatively mild and usually lasts from only a few hours to one day, but in some cases, the complications are severe enough to require hospitalization.

 

Route of entry:

Oral. Consumption of food contaminated with enterotoxigenic S. aureus or ingestion of the preformed enterotoxin.

 

Pathway:

Enterotoxins of Staphylococcus are stable in the gastrointestinal tract.  The enterotoxins once in the intestinal tract, that indirectly stimulate the emetic reflex center by way of undetermined molecular events, which is thought to be the vagus nerve, that involved in the sequence of events that produce the emetic response.

 

Frequency

S. aureus is causing irregular food poisoning around the world, but it is under-reported, as true incidence is unknown for several reasons, including poor responses from victims during interviews with health officials; misdiagnosis of the illness, which may be symptomatically similar to other types of food poisoning (Bacillus cereus emetic toxin causes vomiting similar to enterotoxins), inadequate collection of samples for laboratory analyses, improper laboratory examination etc. However the most important common mistake is that, many of the victims do not seek medical attention because of the short duration of the symptoms.  Staphylococcus food poisoning causes approximately 241,188 infections with 1,064 hospitalizations, and 6 deaths each year in the US according to CDC estimates.

 

Diagnosis

Diagnose is based on the isolation of pre-formed enterotoxin or the isolation of enterotoxigenic staphylococcus from the suspected food consumed and/or the vomitus or feces of the patient. Hence, food history of the patient and rapid onset of symptoms often are sufficient to diagnose staphylococcus food poisoning, where suspected foods are collected and analyzed for presence of viable staphylococcus strains and preformed enterotoxin. However, most conclusive test is to link infection with a specific food, or detection of pre-formed enterotoxin in food sample(s) in case of multiple vehicles exist.

 

Target Populations

All population is believed to be susceptible to Staphylococcus food poisoning, but intensity of symptoms may vary according to the individual patent’s health conditions.

 

Food Analysis

There are several serological methods for detection of pre-formed enterotoxin in foods, which are also utilized to determine the enterotoxigenicity of S. aureus isolate from a food product. Thus, enrichment isolation and direct plating are frequently employed to detect and enumerate S. aureus in foods, where non-selective enrichment is useful for demonstrating presence of injured cells, whose growth is inhibited by selective enrichment media. Hence, Enumeration by enrichment isolation, or selective enrichment isolation, can be determined using either the direct plate count or the most probable number (MPN) of S. aureus in the sample. Currently ELISA-based methods are those most widely used to identify staphylococcal enterotoxins. Several commercially available enzyme-linked immunosorbent assays use both monoclonal and polyclonal antibodies. The intensity of the color reaction or florescence is proportional to the amount of toxin present in the sample.

 

A processed product may be serologically inactive and contains undetectable toxin, but the toxin proteins are highly resistant to treatments where they remains biologically active and can cause infection. Although, the food processing and preservation, including treatment with heat, acidulation, or chemicals, and other treatments kill the live cells and stress the staphylococcal enterotoxin protein. Classical serological methods have been adopted to eliminate viable microorganisms, as in pasteurization or heating, while DNA-based techniques, such as PCR, or direct microscopic observation of the food (if the cells were not lysed), can assist in identification and diagnosis of suspected food products. Staphylococcus species are also diagnosed using Pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST), which are the most common molecular subtyping techniques used for viable staphylococcus isolation from the implicated food, victims, and suspected carriers, such as food handlers.

Reference:

FDA Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins. Second Edition. 2013

Preventive Controls for Human Foods. 2016

www.cdc.gov