Gastrointestinal infections

gastroenteritis - def

inflammation of the stomach and intestines typically caused by infections or intoxications resulting in diarrhoea and vomiting

Caused by enteric pathogens -> bacteria, viruses and protozoa (parasites)

impact of GE in low-middle income countries - list (4)

• Diarrhoeal disease = major cause of morbidity and mortality in children under 5 years old

• Death often due to dehydration -> serious condition especially in children

• Long term effects of repeated infections:

  1. Malnutrition

  2. Growth retardation

  3. Immunodeficiency

impact of GE in higher income countries

Diarrhoeal disease is common but not usually life-threatening -> annoying but mostly self-limiting

causative bacteria - list (11)

• Aeromonas spp.

• Bacillus cereus

• Campylobacter jejuni

• Clostridium perfringens

• E. coli: ETEC, EPEC, EAggEC, EHEC, EIEC

• Salmonella spp.

• Shigella spp.

• Staphylococcus aureus

• Vibrio cholerae

• Vibrio parahaemolyticus

• Yersinia enterocolitica

causative viruses - list (3)

• Rotavirus

• Calicivirus -> norovirus

• Adenovirus

causative parasites - list (3)

• Giardia lamblia

• Cryptosporidium spp.

• Entamoeba histolytica

requirements for microorganism to cause infection

1. Survive passage through stomach

2. Evade host defences to reach intestine

3. Adhere to intestine and multiply to cause damage

• Process takes time so effect of infection is not immediate

Pre-disposing factors that may assist the microbe - list (3)

• Changes in gastric pH -> eg. antacids, surgery etc

• Alterations in normal microbiota of gut that reduce competition →e esp specially important in antibiotic-associated diarrhoea

eg. clostridium difficile

• Protection of acid-sensitive microbe by food -> shields it from stomach acid and allows delayed release after it has passed through acidic areas

Eg. chocolate, cheese

intoxication - def

gastroenteritis caused by toxins produced by pathogens

Toxin can be preformed ormanufactured inside the host by the microorganism -> toxins often heat and acid stable

intoxication - preformed toxins (4)

microorganism contaminates food and multiples -> produces toxin -> toxin ingested with food

Immediate effect on intestinal mucosa -> short incubation period

Main symptom = vomiting

Eg. Staphylococcus aureus and Bacillus cereus

intoxication - manufactured toxins (4)

microorganism manufactures or releases toxins once ingested

Longer incubation period

Main symptom = diarrhoea

Eg. Bacillus cereus, Clostridium perfringens

Incubation period - def

time between exposure to pathogen and onset of disease

incubation period - short

2-6 hours

Suggests ingestion of pre-formed toxin -> food poisoning

Eg. Staphylococcus aureus

incubation period - long

12-48+ hours

Suggests microorganisms need time to multiple and possibly invade causing symptoms

Eg. Salmonella, Campylobacter

infectious dose - def and summary (3)

number of microorganisms needed to cause disease

Some microorganisms have small ID -> typically very acid stable

Eg. Shigella (ID ~10^2 bacteria) and Giardia (ID 10-100 cysts)

Others have large ID -> need to multiply to large numbers to ensure survival though stomach in order to cause disease

• Imperative to adhere to proper post-cooking food handling/ storage

Eg. ETEC (ID ~10^5 - 10^6 bacteria)

symptoms of GE - list (4)

• Diarrhoea

• Vomiting

• Fever

• Abdominal pain

cause of GE symptoms (2)

1. Toxins produced and or damage occurring during growth of microorganisms in body after infection -> microorganisms must multiply after adhering to gut mucosa

Eg. ETEC, EPEC

2. Invasion of intestinal cells by microorganisms -> blood and pus often seen in faeces

Eg. shigella

diarrhoea - def

abnormal faecal discharge with frequence and/ or fluid "stools" -> may contain mucus, pus, blood and or excess fat

Results in forcible expulsion of pathogen but can also aid dissemination

can be watery or bloody

Watery diarrhoea (secretory diarrhoea) - cause

caused by loss of water from tissues due to disruption of mucosal ion pumps

Usually no damage to mucosal cells and little or no inflammation -> affects function of cells (increased salt secretion resulting in influx of water via osmosis)

Eg. Cholera, Rotavirus, ETEC

Dysentery (bloody diarrhoea) - cause

inflammatory disorder of the gastrointestinal tract

Mucosal cells damaged by pathogen or inflammatory response -> prevents water and nutrient absorption and leads to severe damage

Eg. Shigella, Entamoeba histolytica

components of dysentery - list (5)

• Blood

• Possible pus in faeces

• Pain

• Fever

• Abdominal cramps

reservoir of infection - def

• any person, animal, plant, soil or substance in which an infectious agent normally lives and multiples

  • Typically harbours infectious agent without injury to self and serves as source from which other individuals can be infected

  • Infectious agent primarily depends on reservoir for survival

7 “F”s of infection sources - list

• Faeces

• fluids

• fingers

• food

• flies

• fomites

• fornication

faeces as source of infection - humans (3)

When humans are source of infection, referred to as human excretors

Can be asymptomatic infection or recovering from infection → eg. Giardia, Salmonella, Vibrio cholerae in endemic areas

Can be reservoirs of infection → eg. shigella -> human only pathogen

faeces as source of infection - how are human and animal faeces ingested (2)

Contaminated water -> particularly in low-moderate income countries or areas with poor sanitation, or involve pasture run-off

Contaminated food → minced meat is vulnerable to bacterial growth bc increased surface area exposed

Eg. excreting food handler who has poor personal hygiene -> farmer/ butcher who contaminates meat with faecal matter containing pathogen

faeces as source of infection - common bacteria in poultry (2)

Salmonella

campylobacter

faeces as source of infection - common bacteria in cattle (2)

Salmonella

E. coli: EHEC and ETC

faeces as source of infection - common bacteria in pigs (1)

Yersinia

fluids as source of infection - list (3)

1. Drinking contaminated water or consuming contaminated ice in drink → poor sanitation and may result after natural disasters

Eg. cholera (humans)

2. Swimming in water contaminated by human/ animal faeces → pasture runoff

Eg. giardia (humans/ animals)

3. Eating shellfish grown in water which has high numbers of microorganisms

Eg. Vibrio parahaemolyticus

fingers as source of infection - list (3)

1. Transfers microorganisms/ faeces to the mouth or to food -> bacteria/ parasites shed by human excretors

Eg. shigella and giardia when toileting

2. Petting zoos -> especially children

Eg. EHEC

3. Food handlers transferring their normal microbiota

Eg. staphylococcus aureus from nose/ skin -> intoxication, or Salmonella from excretors contaminating food

food as source of infection - list (2)

1. Food associated infections often result via poor food handling/ cooking or contamination of food post-cooking/ lack of refrigeration

2. Food associated intoxications results from ingestion of toxin in food

Eg. Staphylococcus aureus and Bacillus cereus

flies (and other insects) as source of infection - summary

Vectors of enteric pathogens

fomites (inanimate objects) as source of infection - example

cutting boards and water bottles

fornication (sexual activity) as source of infection - summary

Another means of spread of faecal organisms

investigations of GE - small vs large outbreaks

Gastroenteritis is usually self-limiting and self-managed -> typically only outbreaks are investigated for causative organism

Difficult to track source for a single case

Epidemiological investigations when large number of people affected from a point source → different pathogens come from different sources and difference strategies may be needed to prevent further outbreaks

investigations of infections - clues to causative agent (4)

1. What are the symptoms

   • Vomiting suggests food poisoning/ intoxications -> ingestion of pre-formed toxin

   • Copious, watery, bloody or pus diarrhoea

2. What was the incubation period

   • Short IP suggests food poisoning/ intoxication

3. Type of food eaten and food handling/ storage

   • Chicken suggests Salmonella

4. Associated activities

   • Petting zoo, overseas travel, exposure to unhygienic water supply, swimming, communal gathering

investigations of infections - lab procedures summary (4)

collect faeces (rectal swabs from the very ill), food and or vomit → process to provide liquid for exam if needed

1. light microscopy

2. electron microscopy

3. culture

investigations of infections - light microscopy as a lab procedures (4)

wet preparation of faeces/ saline suspension

Examined for red and white blood cells -> pus cells

Iodine stain for cysts, ova and parasites

Special stains depending on strain -> cryptosporidium

investigations of infections - electron microscopy as a lab procedures (3)

useful for viruses

Beam of electrons transmitted through ultra-thin preparation of specimen

Antibodies may be used to clump virus particles together if a particular virus is suspected

investigations of infections - culture as a lab procedures (5)

used extensively in identification of bacterial causes of GE

Media used determined by likely pathogens based on patient history

Routine media = MacConkey and DCA -> Salmonella and Shigella

Specialised media = BP, B. cereus selective agar (BCSA) , CIN, CAMP etc

Enrichment medium = Selenite broth -> Salmonella

features of enterobacteriaceae (6)

• Gram negative rods

• Facultative anaerobes

• Grow on simple media -> eg. NA and MAC

• Oxidase negative

• Fermentative metabolism of glucose

• Motile -> some but not all genera

• Can be differentiated by biochemical tests

features of vibrionaceae (6)

• Gram negative, slender, curved rods

• Facultative anaerobes

• Need 1% salt in media for growth -> found in coastal waters

• Oxidase positive

• Fermentative metabolism

• Motile

features of campylobacteraceae (6)

• Gram negative curved rods (gull wings)

• Microaerophilic -> reduced O2 conditions

• C. Jejuni grows well at 42°C -> higher heat needed to kill bacteria/ prevent bacteria growing in food

• Needs an enriched media containing blood

• Oxidase positive

• Highly motile

bacterial pathogens that can cause GIT disease - enterobacteriaceae (4)

E. coli

Salmonella soo.

shigella spp.

yersinia enterocolitica

bacterial pathogens that can cause GIT disease - vibrionaceae (2)

vibrio cholerae

vibrio parahaemolyticus

bacterial pathogens that can cause GIT disease - campylobacteracaea (1)

campylobacter spp.

bacterial pathogens that can cause GIT disease - other (4)

aeromonas spp.

bacillus cereus → spore producer

clostridium perfringens → spore producer

staphylococcus aureus

identification of causative pathogen process - steps (4)

1. Culture on routine or specialised selective media, then incubate

2. Examine media for characteristic growth of pathogens

3. Choose and perform relevant biochemical tests from Identification Charts in appendix

4. Further characterisation and confirmation via serological testing or stain comparisons

types of media - summary list (6)

1. MacConkey agar (MAC)

2. deocycholate citrate agar (DCA)

3. selenite broth

4. Baird-Parker medium (BP)

5. B. cereus selective agar (BCSA)

6. campylobacter medium (CAMP)

MacConkey Agar - summary (3)

• Routine medium

• Contains bile salts and neutral red and lactose

• Incubated aerobically at 35-37°C

MacConkey Agar - results (2)

• Lactose fermenters produce acid -> pink colonies

Eg. E. coli

• Non lactose fermenters produce alkali metabolites -> yellow/ colourless colonies

Eg. Salmonella and Shigella

deoxycholate citrate agar - summary (4)

• Routine medium

• Highly selective

• Contains bile salts, citrate, neutral red and lactose

• Incubated aerobically at 35-37°C

deoxycholate citrate agar - results (3)

• Lactose fermenters produce acid -> pink colonies

Eg. E. coli inhibited -> may grow poorly

• Lactose non fermenters produce alkali -> yellow/ colour colonies

Eg. Salmonella and Shigella

• Also contains sodium thiosulphate -> H2S producers may grow as colonies with central black dot

Eg. could be Salmonella but not shigella -> Salmonella could still be present without black dot

selenite broth - summary (2)

• Enrichment broth -> used for enrichment of Salmonella spp. In samples of faeces, urine, food, water

• Contains sodium biselenite

  • Inhibits growth of many Gram positive and Gram negative bacteria of normal microbiota

  • Allows Salmonella to increase in numbers during overnight incubation

selenite broth - results (1)

• Brick-red precipitate if Salmonella present -> may not be visible

  • Must subculture onto selective medium (eg. DCA) to detect Salmonella -> look for LNF, then identify

Baird Parker medium - summary (5)

• Specialist medium

• Highly specific and diagnostic medium for Staph aureus

• For isolation and enumeration of Staphylococcus aureus form faeces and food

• Contains egg yolk emulsion and selective agents: tellurite, sodium pyruvate, lithium

• Incubate aerobically at 37°C

Baird Parker medium - results (4)

Staphylococcus aureus:

• Shiny grey/ black colonies -> reduction of tellurite

• Narrow white entire margin -> lipase activity

• Surrounded by a further zone of clearing around narrow white margin -> proteolysis

Even if S. aureus is present, it might not be causative agent -> lots of different strains and not all produce pathogenic toxins

B. cereus Selective Agar - summary (4)

• Specialist medium

• Indicator and diagnostic medium for Bacillus cereus

• Contains egg yolk emulsion, polymyxin B, mannitol, and pH indicator (bromothymol blue)

• Incubate aerobically 37°C

B. cereus Selective Agar - result (2)

B. cereus:

Turquoise/ peacock blue -> mannitol negative

Colonies surrounded by white precipitate -> egg-yolk lecithin hydrolysis

Campylobacter medium - summary (4)

• Specialist medium

• Highly selective and enriched medium -> not diagnostic

• Isolation of Campylobacter spp.

• Contains blood, pyruvate, vitamin B6, antibiotics (to prevent growth of other bacteria)

Campylobacter medium - incubation conditions

Incubate at 42°C for 48 hours in microaerophilic conditions -> specialist gas mix of 5% O2, 10% CO2 and 85% N2

Campylobacter medium - result (2)

Campylobacter:

Shiny, wet-looking colonies with metallic sheen

Spreads along streak lines on plate

3 most serious pathogens possible to investigate first after routine media culture

Salmonella, Shigella, Yersinia → lactose non-fermenters

not normal flora in gut and even low numbers present is significant

types of tests - list (4)

• biochemical

• serological

• phage typing

• molecular methods - PCR

biochemical tests - summary (2)

Can be used to screen suspicious colonies

Tests selected on basis of characteristic growth on routine and specialised media

biochemical tests - examples (6)

• Oxidase

• Oxidation/ fermentation

• Lactose fermentation

• Urease

• H2S production

• Motility

serological tests - types (2)

agglutination

enzyme-immuno-assay (EIA/ ELISA)

serological tests - agglutination (2)

Bacteria and antibody directed against the bacterium -> visible clump of bacteria

Eg. characterisation of Salmonella spp. Isolated from faeces

serological tests - Enzyme immuno assay

Bacteria binds to antibody -> antibody loaded with indicator binds to bacteria if present -> colour change if present

phage typing - summary (3)

Test susceptibility to a panel of bacterial viruses -> bacteriophages

Can be used for Salmonella spp. And Staphylococcus aureus strains

Preliminary conclusions may be drawn on results -> same phage type of bacteria in food and person's faecal sample may indicate bacteria in food was source of person's infection → more definitive proof now required

eg. using molecular methods like PCR and WGS

phage typing - steps (5)

1. Lawn culture of abcteria applied to plate

2. Panel of phage dotted on plate in grid pattern

3. Incubate

4. Read pattern of lysis

5. Phage type assigned depending on lysis pattern

molecular patterns (PCR) - summary

PCR used to look for genes encoding virulence determinants of enteric pathogens in patient isolates

Staphylococcus aureus - IP, source, ID

Short IP -> 2-6 hours

Source = normal flora of humans -> esp skin and nose

Typically involves foods high in sugar or salt

Eg. cut, slice, grated mixed or ground food which has been left at RT -> selectively favour growth of staphylococci

Infectious dose = at least 10^5 bacteria/ g food

Staphylococcus aureus - toxins summary (4)

Production of heat stable enterotoxin in food -> produced by ~50% of S. aureus strains

6 toxigenic types -> A, B, C, D, E, G

Very small quantities of toxin case illness -> detect toxins in faeces by enzyme-immuno-assay for definitive diagnosis

Toxin binds to neural receptors in upper GIT

Bacillus cereus (emetic) - IP, source and spore summary

Short IP -> 1-5 hours

Clear association with consumption of fried rice/ starchy foods

Spores contaminate food and survive boiling and germinate during slow cooling at room temp

Vegetative cells grow and produce toxin -> Cereulide peptide

Subsequent flash-frying to make fried rice does not destroy heat stable enterotoxin

Bacillus cereus (emetic) - toxin summary

Emetic toxin induces vomiting and abdominal cramps -> can cause diarrhoea in 1/3 cases

Mild and self-limiting

Bacillus cereus (diarrhoeal) - IP, source, ID

Longer IP -> 6-15 hours

Associated with consumption of meat and dried vegetables, soups, milk products containing spores → germinate post cooking and grow to significant numbers if food not properly refrigerated

Infectious dose = at least 10^5 bacteria/g of food ingested

Bacillus cereus (diarrhoeal) - toxins summary

Bacteria produce heat, acid and protease labile toxin/s in intestine

Predominately haemolysin BL (HBL) and or non-haemolytic enterotoxin and or cytotoxin K

Causes intestinal fluid secretion by activation of adenylate cyclase enzymes → watery diarrhoea, abdominal cramps and vomiting in 1/4 cases

Mild and self limiting

clostridium perfringens - IP, source, ID

Longer IP -> 6-24 hours

Spores found in soil, dust, gut of animals and humans

Foods containing spores are cooked then slowly cooled for extended time (not refrigerated) -> spores survive and germinate and grow during slow cooling at room temp

Eg. beef, poultry, fish, sauces and gravies

Infectious dose of 10^6-10^7 -> eat large numbers of bacteria in re-warmed food

clostridium pergringens - spores and toxins summary

Ingested bacteria sporulate once in stomach and release heat-labile enterotoxin in intestine

Toxin inhibits glucose transport, damages intestinal epithelium and causes protein loss in lumen

Results in watery diarrhoea and cramping abdominal pain

Self-limiting -> 24 hours or less

epidemiology - def

study of occurrence, spread and control of diseases

epidemiologists - role

collect data to try to determine cause and mode of transmission of a disease and to design control measures to limit further spread

endemic - def

occurs in community or area at all times or regularly

epidemic - def

occurrence of more cases of a disease than expected ina. given area or amount a particular group of people over a particular period of time

pandemic - def

an epidemic occurring over a very wide area and usually affecting a large proportion of the population

case definition - def

set of criteria for classifying whether a person has a particular disease

principles of outbreak investigation - summary steps (4)

1. describe outbreak

2. develop test and hypothesis about cause of outbreak

3. intervene to control the outbreak

4. implement appropriate mechanisms to prevent future outbreaks

principles of outbreak investigation - features of describing the outbreak (4)

1. time

2. place

3. person

4. pathogen

principles of outbreak investigation - develop and test hypotheses about cause of outbreak (2)

1. perform analytic epidemiology studies

2. microbiological investigations

methods of controlling food-borne GE (3)

1. collaborative bodies → state, national and international health departments

2. implementation of paddock-to-plate strategies → HACCP = internationally recognised and used food management standard

3. other → consumer education, irradiation of food during production

WHO rules for safe food preparation (10)

1. Choose foods processed for safety

2. Cook food thoroughly

3. Eat cooked foods immediately

4. Store cooked foods carefully

5. Reheat cooked foods thoroughly

6. Avoid contact between raw foods and cooked foods

7. Wash hands repeatedly

8. Keep all kitchen surfaces meticulously clean

9. Protect food from insects, rodents and other animals

10. Use safe water

challenges to controlling food-borne GE (5)

• Food production and distribution networks = incredible large and complex

• Many food industries are now globalised -> imports and exports from countries with different standards for food production or distribution mechanisms

• Patterns of food consumption change -> new food items to consider as food trends rapidly change

• Vulnerable groups -> food must be safe for all or most people to consume including immunocompromised or adequate warnings must be provided

• Use of antibiotics in animals for food production can lead to antibiotic resistant microbes -> potential for contamination

global impacts of GE in upper middle income countries (4)

Comparatively low levels of endemic disease though occasional outbreaks occur -> usually not life-threatening

Expectations for cheap but safe food

Trade with other countries

Protected by regulation and litigation

global impacts of GE in upper middle income countries (3)

Diarrhoeal disease still major cause of morbidity and mortality in children under 5

Repeated morbidity in children under 5 can lead to malnutrition and diarrhoea cycles

80% of deaths occur in children under 2 -> mostly dehydration as well as dysentery and malnutrition

main impacts of GE in Aus (3)

1. cost to individuals, businesses, healthcare and gov

2. hospitalisations

3. deaths

strategies to prevent GE (7)

• Access to safe drinking water

• Use of improved sanitation

• Hand washing with soap

• Exclusive breastfeeding for the first 6 months of life

• Good personal and food hygiene

• Health education about hot infections spread

• Rotavirus vaccination