Introduction to Microbiology

Non-Invasive Enteritis and Food Poisoning

1FOODBORNE ILLNESS (Bacterial)Foodborne illness results from eating food contaminated with organisms or toxins

Foodborne illness tends to occur at picnics, school cafeterias, and large social functions

These are commonly situations in which food may be left unrefrigerated or food preparation techniques are insufficiently safe

Foodborne illness often occurs from undercooked meats or dairy products that have remained at room temperature for extended periods2In patients with foodborne illness, fluid consumption is important to avoid dehydration

Solid foods should not be eaten until the diarrhea has passed, and dairy should be avoided, as it can worsen diarrhea temporarily

Intravenous fluid may be indicated in patients with severe diarrhea who are unable to drink fluidsfor example, caused by nausea or vomiting

3Most patients spontaneously recover from the most common types of foodborne illness within a couple of days

Antibiotic therapy is usually not indicated, except in cases of severe illness

Infants and elderly people have the greatest risk for foodborne illness

It is estimated that foodborne gastroenteritis causes 48 million illnesses, 127,000 hospitalizations, and 3,000 deaths in the United States annually4VIBRIOMembers of the genus Vibrio are short, curved, rod-shaped organisms

Vibrios are closely related to the family Enterobacteriaceae

They are rapidly motile by means of a single polar flagellum

Vibrios are facultative anaerobes5Pathogenic vibrios include

1) Vibrio cholerae, serogroup O1 strains that are associated with epidemic cholera

2) non-O1 V. cholerae and related strains that cause sporadic cases of choleralike and other illnesses

3) Vibrio parahaemolyticus and other halophilic vibrios, which cause gastroenteritis and extraintestinal infections6A. EpidemiologyV. cholerae is transmitted to humans by contaminated water and food

Among humans, long-term carriage is considered uncommon

Outbreaks have been associated with raw or undercooked seafood harvested from contaminated waters

Natural (and even man-made) disasters are often followed by cholera outbreaks7B. PathogenesisFollowing ingestion, V. cholerae infects the small intestine

Adhesion factor(s) are important for colonization and virulence

The organism is noninvasive but adheres to the epithelium by expression of pili called Tcp, or toxin-coregulated pili

These pili are coordinately expressed along with cholera toxin, which is an enterotoxin that initiates an outpouring of fluid

This, in turn, causes an outflowing of ions and water to the lumen of the intestine8C. Clinical significanceFull-blown cholera is characterized by massive loss of fluid and electrolytes from the body

After an incubation period ranging from hours to a few days, profuse watery diarrhea (rice-water stools) begins

Untreated, death from severe dehydration causing hypovolemic shock may occur in hours to days, and the death rate may exceed 50 percent

Appropriate treatment reduces the death rate to less than 1 percent

Patients with suspected cholera need to be treated prior to laboratory confirmation, because death by dehydration can occur within hours9D. Laboratory identificationV. cholerae grows on standard media such as blood and Mac-Conkey agars

The organism is oxidase positive, but further biochemical testing is necessary for specific identification of V. cholerae10E. Treatment and preventionReplacement of fluids and electrolytes is crucial in preventing shock and does not require bacteriologic diagnosis

Antibiotics (doxycycline is the drug of choice) can shorten the duration of diarrhea and excretion of the organism

Prevention relies primarily on public health measures that reduce fecal contamination of water supplies and food

Adequate cooking of foods can minimize transmission

Vaccines that are only modestly protective are available in many other countries but not in the United States11ESCHERICHIA COLIEscherichia coli is part of the normal flora of the colon in humans and other animals but can be pathogenic both within and outside of the GI tract

E. coli has pili that are important for adherence to host mucosal surfaces, and different strains of the organism may be motile or nonmotile

Most strains can ferment lactose in contrast to the major intestinal pathogens

E. coli produces both acid and gas during fermentation of carbohydrates12A. Structure and physiologyE. coli shares many properties with the other Enterobacteriaceae

They are all facultative anaerobes, they all ferment glucose, and they all can generate energy by aerobic or anaerobic respiration13B. Clinical significance: intestinal diseaseTransmission of intestinal disease is commonly by the fecaloral routecontaminated food and water serving as vehicles for transmission

E. coli infection should be suspected in all patients with acute bloody diarrheaparticularly if associated with abdominal tenderness and absence of fever14C. Clinical significance: extraintestinal diseaseThe source of infection for extraintestinal disease is frequently the patient's own flora, in which the individuals own E. coli is nonpathogenic in the intestine

However, it causes disease in that individual when the organism is found, for example, in the bladder or bloodstream (normally sterile sites)15D. Laboratory identification1. Intestinal disease

Because E. coli is normally part of the intestinal flora, detection in stool cultures of disease-causing strains is generally difficult

Current molecular techniques, such as polymerase chain reaction, may be employed to identify E. coli strains162. Extraintestinal disease

Isolation of E. coli from normally sterile body sites is diagnostically significant

Specimens may be cultured on MacConkey agar

Strains of E. coli can be further characterized on the basis of serologic tests17E. Prevention and treatmentIntestinal disease can best be prevented by care in selection, preparation, and consumption of food and water

Maintenance of fluid and electrolyte balance is of primary importance in treatment

Antibiotics may shorten duration of symptoms, but resistance is nevertheless widespread

Extraintestinal diseases require antibiotic treatment

Antibiotic sensitivity testing of isolates is necessary to determine the appropriate choice of drugs1819Staphylococcal gastroenteritisCaused by ingestion of food contaminated with enterotoxin-producing S. aureus

Often contaminated by a food handlerThese foods tend to be protein rich (e.g. egg salad or cream pastry) salty (S. aureus is salt tolerant)improperly refrigerated

These heat-resistant toxins are able to withstand subsequent reheating

20Symptoms, such as nausea, vomiting, and diarrhea, are acute followinga short incubation period (less than 6 hours)triggered by the toxin on the GI tract rather than from infection

Staphylococcal food poisoning has short incubation periodthe toxin in the food has already been formed before the food is ingested

Laboratory IdentificationIdentification of an isolate as a staphylococcus relies largely onmicroscopic and colony morphologycatalase positivity

Bacteria stain strongly gram-positivefrequently seen in grapelike clusters

S. aureus colonies tend to be yellow

S. aureus is also distinguished from most coagulase-negative staphylococci by being mannitol-positive22

Gram stain of S. aureus cells which typically occur in clusters

Scanning electron micrographof S. aureus; false color addedLaboratory IdentificationSpecimensStool, vomitus, and food can be tested for toxin and cultured if indicatedCultureMannitol salt agar

Second, the isolate is cultured on mannitol salt agar, which is a selective medium with 79% NaCl that allows S. aureus to grow, producing yellow-colored colonies as a result of mannitol fermentation and subsequent drop in the medium's pH.24ImmunityS. aureus infections do not elicit strong or long-lasting immunity

as demonstrated by the continuing susceptibility of individuals to S. aureus infections throughout life25TreatmentSerious S. aureus infections require aggressive treatment including systemic antibiotics

Choice of antibiotics is complicatedby the frequent presence of acquired antibiotic resistance determinants

Virtually all community and hospital-acquired S. aureus infections are now resistant to penicillin G

This has required the replacement of the initial agent of choice, penicillin G, by -lactamase-resistant penicillins, such as methicillin or oxacillin

However, increased use of methicillin and related antibiotics has resulted in S. aureus that is resistant to a number of -lactam antibiotics, such as methicillin, oxacillin and amoxicillin

These strains are known as methicillin-resistant S. aureus (MRSA)26PreventionThere is no effective vaccine against S. aureus

Infection control proceduresdisinfection of hands and fomitescontrol of nosocomial S. aureus epidemics27Clostridium perfringensC. perfringenslargenonmotilegram-positiveencapsulated bacillus

It is ubiquitous in nature, with its vegetative form as part of the normal flora of the GI tract

Its spores are found in soilspores are rarely seen in the body or following in vitro cultivation

Some strains of C. perfringens cause a common form of food poisoningC. perfringens can cause anaerobic cellulitis and myonecrosis (gas gangrene)

Anaerobic cellulitis: Cellulitis actually means "inflammation of the cells." Specifically, cellulitis refers to an infection of the tissue just below the skin surface.

Myonecrosis: The destruction or death of muscle tissue28C. perfringens Foodborne InfectionC. perfringens is a common cause of foodborne infection in the United States

Typically, the onset of nausea, abdominal cramps, and diarrhea occurs 8 to 18 hours after eating contaminated foodfever is absent and vomiting rareattack usually self-limited, with recovery within 1-2 daysoccurrence of clinical symptoms requires large inoculum

An enterotoxin (not to be confused with endotoxin) is a protein exotoxin released by a microorganism that targets the intestines. Its target site is indicated within its name.29Typical clostridial enterotoxin food poisoningcooking that fails to inactivate sporesfollowed by holding the food for several hours under conditions that allow bacterial germination and several cycles of growth

Vegetative cells are consumed in the contaminated productC. perfringens then reproduces following ingestion and produces toxin in vivo

Meats, meat products, and gravy are the most commonly implicated foods in C. perfringens foodborne illness

Laboratory identificationIn food infection, the organism can be sought in suspected food and the patient's feces

Gram stain and other laboratory findings greatly help planning of antibiotic therapy in patients with clinical manifestations of gas gangrene31

Photomicrograph of gram-positive Clostridium perfringens bacilli

Cultures of Clostridium perfringens grown on an egg yolk agar plateTreatment and preventionC. perfringens is sensitive to penicillin and several common inhibitors of prokaryotic protein synthesis

Because clostridial infections usually involve a mixture of species, the use of broad-spectrum antibiotics is appropriate33Clostridium botulinumC. botulinum causes botulism, which occurs in several clinical forms

Botulism is caused by the action of a neurotoxin that is one of the most potent poisons known and causes a flaccid paralysis

Contact with the organism itself is not required, and the disease can be solely due to ingestion of toxin-contaminated foodTheClostridium botulinumbacteria are the cause ofbotulism. Vegetative cells ofC. botulinummay be ingested. Introduction of the bacteria may also occur viaendosporesin a wound. When the bacteria arein vivo, they induce flaccid paralysis. This happens becauseC. botulinumproduces a toxin which blocks the release ofacetylcholine. Botulism toxin blocks the exocytosis of presynaptic vesicles containing Acetylcholine (ACh). When this occurs, the muscles are unable to contract. Other symptoms associated with infection from this neurotoxin include double vision, blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, and muscle weakness. Botulism prevents muscle contraction by blocking the release of ACh, thereby halting postsynaptic activity of the neuromuscular junction. If its effects reach the respiratory muscles, then it can cause respiratory failure, leading to death.34

Clostridium botulinum stained with gentian violetEpidemiologyC. botulinum is found worldwide in soil and aquatic sedimentsspores frequently contaminate vegetables and meat or fish

Under appropriate conditions(strictly anaerobic environment, and neutral/alkaline pH)the organism germinatestoxin is produced during vegetative growth

Because the toxin is often elaborated in food, outbreaks frequently occur in families or other eating groupsAquatic sediments: Elaborated: Developed or present36Clinical significance:a. Classic botulism (Children and Adults)Food poisoning in which a patient first begins to experiencedifficulties infocusing visionswallowingother cranial nerve functions12 to 36 hours after ingesting toxin-containing foodthere is no fever or sign of sepsisCranial nervesarenervesthat emerge directly from thebrain, in contrast tospinal nerves37A progressive paralysis of striated muscle groups develops

Mortality rate is about 15 percent, with the patient usually succumbing to respiratory paralysis

Recovery, which involves regeneration of the damaged nerves, is protracted, lasting several weeks

Striated muscle tissueis a form offibersthat have repeatingsarcomeres. More specifically, it can refer to:

Cardiac muscle(cardiac refers to theheart)Skeletal muscleBranchiomeric muscles(embryologically different from skeletal muscle)

Succumbing: To submit/dieProtracted: Extended38b. Infant botulismThe most common form of botulism in the United States today is infant botulism

An infant has yet to develop mature colonic microbial floraC. botulinum can colonize the large bowel of infants and produce toxin

The botulinum toxin is produced in vivo and slowly absorbedConstipation, feeding problems, lethargy, and poor muscle tone are common early signs

Lethargy: A state of sluggishness, inactivity, and apathy39Supplementation of infant foods with raw honeywhich is contaminated with C. botulinum spores, may transmit the organism

The condition is possibly a cause of Sudden Infant Death Syndrome (SIDS)recovery is the usual outcomefollowing symptomatic treatment that may be prolonged

Laboratory identificationThe organism can be culturedand identified by standardanaerobic methods

Toxin is also identifiablein serum, stool, and food41Treatment and preventionAntitoxin, which neutralizes unbound botulinum toxin, should be administered as soon as possible in suspected botulinal intoxication

Supportive measures, including mechanical ventilation, may be required

In wound and infant botulism, the infection can be treated with penicillin or other antibiotics to which the organism is sensitive

The toxin is inactivated at boiling temperatures, although killing of botulinal spores requires moist heat under pressure (autoclaving)42