Salmonella: Infections, Distribution, and Salmonellosis

• Food borne infections are highest in the summer months.
• They are caused by various types of Salmonella, which differ by country.
• Common Salmonella types include S. enteritidis and S. typhimurium.
• Salmonella are Gram negative rods, non-spore forming, related to E. coli, with over 2000 strains.
• There are approximately 40,000 cases of Salmonella in the US annually, resulting in 1,000 deaths.

Distribution of Salmonella

• The primary habitat for Salmonella is the intestinal tract of mammals, birds, reptiles, and insects.
• Salmonella is excreted in faeces and spread by hand, polluted water, and insects.
• People can be asymptomatic carriers of Salmonella.
• It is found in raw poultry, eggs, products containing eggs, and sometimes on unwashed salads and fruits.
• Any food can become cross-contaminated with Salmonella.
• It frequently occurs after handling pets, especially reptiles.

Salmonellosis

• Symptoms of Salmonellosis occur 12 hours to 3 days after infection.
• Symptoms include diarrhoea, fever, abdominal cramps, and headache, and may also include nausea, loss of appetite, and vomiting.
• Symptoms typically last 4 to 7 days.
• Diarrhoea may be severe and require hospitalisation.
• Salmonellosis normally clears up on its own, but sometimes may spread to the blood, requiring antibiotics.
• Approximately 3% of infected people become carriers of the organism.
• Carriers are recommended not to serve or prepare food for others.
• Complications may include Reiter’s syndrome, which can lead to arthritis and painful urination.

Conditions for outbreaks

• Food must contain viable Salmonella at high levels.
• Food must be capable of sustaining Salmonella growth.
• Sufficient time must be available for Salmonella to increase in number in food.
• The organism must be ingested in viable form.

Toxins and Virulence

• Salmonella's ability to infect involves the production of an enterotoxin and a cytotoxin.
• These toxins make cell membranes permeable, allowing bacteria to invade cells.

Incidence and Vehicle Foods

• Salmonella outbreaks occur at large banquets and fast food outlets.
• Vehicle foods include pasteurised milk, potato salads, raw fish, and foods with eggs like mayonnaise and ice cream.
• Many Salmonella isolates are antibiotic resistant, such as S. typhimurium Definitive type 104 (DT104).
• Most Salmonella are sensitive to heat.

Typhoid Fever

• Typhoid fever is caused by Salmonella typhi and S. paratyphi.
• It is common in developing countries, affecting 12.5 million people each year.
• Typhoid fever is associated with contaminated water and food handlers.
• It involves infection of the lymph system, then most major organs including the intestine.
• Typhoid fever is life-threatening.
• Vaccination is available for travellers.

Typhoid Mary

• Mary Mallon was an Irish immigrant who worked as a cook in New York City.
• Typhoid outbreaks occurred where she worked between 1906 and 1915.
• She was identified as a carrier of S. typhi.
• Mary was quarantined on an island for 3 years, then released after pledging not to work as a cook, handle food, or report to public health authorities every 3 months.
• She disappeared, changed her name, and began cooking again, and was recaptured 5 years later following major epidemics.
• Mary was imprisoned in a house in NY for 23 years until her death in 1938.
• She was associated with 47 illnesses and 3 deaths.

Preventing Salmonella diseases

• Prevent infection of poultry by avoiding contaminated feed.
• Contain infection when it arises on farms.
• Maintain hygienic control of evisceration at slaughter.
• Refrigerate slaughtered carcasses rapidly.
• Pasteurise or sterilise foods.
• Thoroughly cook food.
• Completely thaw food prior to cooking.
• Employ good hygiene practices.
• Only allow healthy people to handle food.
• Avoid eating undercooked foods, particularly eggs and chicken.
• Avoid eating food from street vendors.

E. coli

• E. coli is a coliform bacterium found in the gut of warm-blooded animals like cattle, sheep, and goats.
• It is Gram negative and Oxidase negative.
• There are hundreds of strains of E. coli.
• Non-virulent strains are usually present in the gut, helping produce vitamins and suppress the growth of harmful bacteria.
• Virulent strains can cause severe foodborne illness that can be fatal.

Serotyping

• Gram-negative bacteria have lipopolysaccharide (LPS) in their cell wall
• LPS is antigenic.
• There are many different types of LPS called "O" antigens, numbered as they were discovered.
• E. coli are motile and have flagella.
• Flagella are antigenic and called H antigens, numbered in sequence as they were discovered.
• It is possible to distinguish E. coli strains into different groups based on O and H antigens.
• The O antigen identifies the serogroup, and the H antigen identifies the serotype.
• E. coli isolates can have a variety of antigen combinations, with 30 serovars responsible for diarrheal disease.
• Serotyping is used for other types of bacteria as well.
• Strains with particular virulence often belong to particular serotypes.
• It is not the O or H antigens that account for virulence.
• Not all strains of a particular serotype (e.g. E. coli O157, H7) are virulent.

Virotypes

• Virotyping is based on the presence of certain virulence factors and their interaction with mammalian cells or tissues.
• Pathogenic E. coli are divided into 6 virotype groups: ETEC, EPEC, EHEC, EIEC, EAEC, and EDEC.

ETEC

• ETEC stands for Enterotoxigenic E. coli.
• It causes diarrhoea in infants and in less developed countries.
• ETEC is also known as "Traveller's diarrhoea" or "Delhi Belly."
• It produces two types of toxin: LT (heat-labile toxin) and ST (heat-stable toxin).
• LT resembles cholera toxin.
• Symptoms include watery diarrhoea, vomiting, and severe dehydration.
• ETEC may be lethal in young children.

EHEC

• EHEC stands for Enterohaemorrhagic E. coli.
• It is also known as STEC (Shiga-toxigenic E. coli) and VTEC (Verotoxigenic E. coli).
• EHEC makes a toxin similar to Shiga toxin produced by Shigella.
• It can cause mild gastroenteritis to haemorrhagic colitis and Haemolytic Uremic Syndrome (HUS).
• EHEC is prevalent in developed countries.
• It can be fatal.
• The most common serotype associated with VTEC is O157:H7.
• Serotype O104:H4 was responsible for the German outbreak of VTEC in 2011, associated with organic bean sprouts, resulting in ~800 cases of HUS, 2,300 cases of non-HUS, and 31 deaths.
• EHEC is often present in the gut of cows, but is harmless to cows.
• It can contaminate milk, meat, and vegetables if contaminated manure is used as fertiliser.
• Undercooked minced beef is implicated in many outbreaks.
• A 2006 outbreak in the US (200 sick, 3 deaths) was associated with spinach and lettuce or hamburgers.
• A 1993 outbreak in the US (500 sick, 4 dead) was associated with hamburgers.

Irish Data

• Cases of Enterohaemorrhagic E. coli in Ireland between 2015 and 2019

• 2015: 730 cases

• 2016: 839 cases

• 2017: 923 cases

• 2018: 926 cases

• 2019: 988 cases

E. coli O157:H7

• E. coli O157:H7 is an emerging cause of foodborne illness.
• There are an estimated 73,000 cases in the US each year (CDC), with infection fatal in ~61 cases per year in the US.
• It was first recognised in 1982, associated with undercooked contaminated burgers.
• Contaminated meat looks and smells normal.
• The incubation period is 2 to 4 days, and the illness normally lasts 5 to 10 days.
• The infective dose is unknown, but may be as low as 10 organisms.
• Symptoms include haemorrhagic colitis, bloody diarrhoea, fever, vomiting, and haemolytic uremic syndrome (HUS) in the old and young.
• The illness lasts about 10 days.
• Possible contaminants include ground beef, raw milk, chicken, apple cider, municipal water, person-to-person transfer, and asymptomatic carriers.

Prevention

• Thoroughly cook meat, particularly ground beef.
• Eat burgers while hot.
• Avoid handling meat with bare hands.
• Drink only pasteurised milk and fruit juices.
• Follow good hygiene practices on the farm and in the slaughterhouse.
• Do not buy cooked foods in butcher shops.
• Do not fertilise land or crops with cattle manure.

Diagnoses

• Detection requires the use of Sorbitol-McConkey (SMAC) agar.
• Strain O157:H7 is sorbitol negative, and 95% of E. coli isolates are sorbitol positive.
• Rapid ELISA assays are available.
• The bacterium must be detected in the stool of the victim.

Treatment

• Most people recover within 10 days without antibiotic therapy.
• There is no evidence that antibiotics limit the disease, and there are possibilities that antibiotics contribute to kidney complications.
• Antidiarrheal agents are sometimes used but should be avoided.
• HUS (Haemolytic Uremic Syndrome) is treated in intensive care with blood transfusions and kidney dialysis.
• 3 to 5% of HUS patients die.

Long term consequences

• 1/3 of HUS patients have prolonged kidney complications requiring long-term dialysis.
• 1/10 of HUS patients have lifelong high blood pressure, seizures, some form of paralysis, and bowel complaints, often necessitating removal of part of the bowel.

Listeria monocytogenes

• Listeria monocytogenes is a Gram-positive bacterium.
• It is non-sporeforming and rod-shaped.
• It exhibits tumbling motility at 25°C.
• Listeria is capable of growing inside cells of a host animal (an intracellular parasite), growing within the nucleus and cytoplasm once phagocytosed, and can infect and multiply in nonphagocytic cells also.
• It can reproduce at refrigeration temperatures.
• They are found on some vegetables as soil contaminants.

Listeriosis

• Listeriosis is a serious but rare infection caused by consuming food contaminated with Listeria monocytogenes or through the placenta.
• The incubation period is approximately 3 weeks (3–70 days).
• Symptoms include fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhoea.
• If it spreads to the nervous system, it can cause meningitis and/or encephalitis.
• It may also cause miscarriage or stillbirth in pregnant women.
• Treatment involves antibiotics.
• The infective dose may be as low as 1,000 bacteria.

Meningitis

• Meningitis has a sudden onset.
• Symptoms include severe headache, dizziness, stiff neck or back, loss of balance, and convulsions/coma.
• Without therapeutic intervention, death usually ensues.

Fatality and Occurrence

• Listeriosis has a fatality rate of approximately 20–30%.
• In the U.S., there are approximately 2,500 cases each year, with 500 deaths.
• Irish data usually show less than 10 cases per year.

Virulence Properties

• Pathogenic strains produce b-hemolysins on blood agar.
• The haemolytic protein produced by these strains is listeriolysin O.
• It is a 60 kDa protein produced during exponential growth.
• It enables the microbe to enter non-phagocytic cells.

Who is at risk?

• Anyone can become ill, but those most at risk are the immunocompromised, pregnant women

Listeriosis during pregnancy

• Pregnant women are 20 times more likely than other healthy individuals to get listeriosis.
• Approximately one-third of Listeriosis cases occur in pregnant women.
• Infected pregnant women may be completely asymptomatic or only suffer mild flu-like symptoms.
• Symptoms in the last half of pregnancy include sudden chills, fever, sore throat, headache, lower back pain, discoloured urine, and diarrhoea.
• Infection during pregnancy can lead to miscarriage, premature delivery, stillbirth, and perinatal septicaemia or meningitis.

The immunocompromised

• Persons with weakened immune systems, cancer, diabetes, or kidney disease are at risk.
• Persons with AIDS are almost 300 times more likely to get Listeriosis than other healthy individuals.
• Persons who take glucocorticosteroid medications are also at risk.

Prevention

• Listeria is killed by pasteurisation and cooking.
• Contamination may occur after cooking and before packing (e.g., hot dogs, deli meats).
• Guidelines for prevention are similar to those for other foodborne illnesses.
• General recommendations include not drinking unpasteurised milk, thoroughly cooking food, separating raw from cooked food, and refrigerating perishable items.
• Special recommendations for those at risk include not eating hot dogs or deli meats unless they are reheated until steaming, avoiding getting meat juices on other foods or utensils, not eating soft cheeses unless clearly made from pasteurised milk, not eating refrigerated patés or meat spreads, and not eating smoked seafood.

Vehicle of Infection

• Summer of 1983, 49 patients in Massachusetts acquired listeriosis.
• 7 cases were in fetuses or infants, and 42 were in immuno-suppressed adults.
• The illness was found to be associated with drinking a specific brand of pasteurised whole milk.
• Milk came from farms with mastitis in cattle.
• Pasteurisation procedures were checked and found to be working at the dairy in question.