Gram negative bacteria Flashcards
what makes bacteria gram negative?
Cell wall contains lipid A
clinical manifestations associated with the lipid A in cell wall
fever, vasodilation, inflammation, shock and DIC (disseminated intravascular coagulation)
DIC (disseminated intravascular coagulation)
formation of blood clots within blood vessels throughout the body.
Enterobacteriaceae found in humans and environment
Enteric bacteria- members of intestinal microbiota of humans and animalsUbiquitous in water, soil and decaying vegetation
structure of Enterobacteriaceae
Coccobacilli or bacilli – 1μm x 1.2-3μmIf motile – peritrichous flagellaSome have prominent capsule, others loose slime
physiology of Enterobacteriaceae
All reduce nitrate to nitriteFerment glucose anaerobically – although grow better aerobicallyAll oxidase negative
Enterobacteriaceae Pathogenicity
Outer membrane lipopolysaccharide – 3 main antigenic componentsa) core polysaccharide shared by all enteric bacteria-common antigenb) O polysaccharide- various antigenic varieties among strains and species e.g. Salmonella spc) Lipid A
virulence factors x7
Lipid A
Capsules
Fimbriae
Exotoxins
Iron binding compounds
Haemolysins
Type III secretion system
Opportunistic coliforms x6
E.coli
Klebsiella sp
Serratia sp
Enterobacter sp
Hafnai sp
Citrobacter sp
Opportunistic noncoliform x4
Proteus spMorganella spProvidencia spEdwardsiella sp
Truly pathogenic Enterobacteriaceae x3
Salmonella spShigella spYersinia spNot considered members of normal microbiota of humans – almost always pathogenic due to their virulence factorsAll 3 synthesize type III secretion systems
salmonella characterics
Gram-negative facultatively anaerobic predominantly motile by peritrichous flagellaover 2200 strains identifiedall species are pathogenic to both humans and other animalscomplex cycle of transmission
four main reservoirs of salmonella bacteria
intestinal tract of birds and animalssewage, fertilisers and slurryanimal feedshuman carriers
foods typically associated with salmonella contamination
meat and meat productsmilk and milk productseggs and egg productsfishconfectionerymiscellaneous foods - dried yeast, frogs legs, marijuana, peanut butter
clinical manifestation of Enteritis (salmonella infection caused) x6
diarrhoea, abdominal pain, mild fever, chills, nausea, vomiting
incubation period and infection dose of salmonella
incubation period of 5-72 hours, but occasionally up to 7 dayslasts 2-5 daysinfective dose varies from as little as 50 cells to 1,000,000 per gram of food
Enteric Fevers
some species can cause more serious infections eg. S. typhi and S. paratyphi.
pathogenicity of salmonella
Salmonellae adhere to the epithelial lining of the ileum by means of fimbriae followed by invasion and multiplication
toxins produced salmonella
endotoxin3 enterotoxinscytotoxin
prevention of salmonella outbreaks
joint action by agriculture and food sectors by the consumer control starts on the farm and continues through the food chain to the consumer
S.typhi infection
Humans sole hostsInfection via ingestion of contaminated food or water
spread of S.typhi infection through body
Bacteria pass through intestinal wall to bloodstreamPhagoscytised but not killed and carried to liver, spleen, bone marrow & gallbladder
clinical manifestations of S.typhi infections
Patients have increasing symptoms of fever, headache, malaise, muscle pain & loss of appetite – lasts about a weekBacteria released from gallbladder to re-infect the intestines – resulting in gastroenteritis abdominal pain & recurring bacteremiaIn some bacteria ulcerate and perforate intestinal wall causing peritonitis in abdominal cavity.
Escherichia coli taxonomy
Domain- Bacteria Phylum- Proteobacteria Class- Gammaproteobacteria Order- Enterobacteriales Family- Enterobacteriaceae
diseases associated by E coli x4
UTI’sNeonatal meningitisGastroenteritisSevere to fatal hemorrhagic colitis
Gastroenteritis caused by e coli
exotoxin called enterotoxin bind proteins on intestinal tract cell lining portion enters cell and triggers a series of chemical reactions results in loss of electrolytes producing watery d & v – a common cause of paediatric infections on developing countries.
hemorrhagic colitis
enterohemorrhagic E coli, or EHEC - most notably O157:H7produce relatively large amounts of the bacteriophage-mediated Shiga-like toxin. This toxin is called Vero toxin (VT), or Vero cytotoxin after its cytotoxic effect on cultured Vero cells. Many strains of O157:H7 also produce a second cytotoxin (Shiga-like toxin 2, or Vero toxin 2), which is similar in effect but antigenically different.
treatment of e coli infections
supportive treatment - dehydration various antimicrobials ( carefully selected as some will worsen the symptoms as lysed bacteria release exotoxin )
prevention of e coli infection
prevent faecal contamination of food and water good personal hygiene
shigella
Genus of Gram-negative, non-spore forming rod-shaped bacteria closely related to Escherichia coli and Salmonella.
shigellosis
caused by shigella infection only in primates
Serogroup A: S. dysenteriae (12 serotypes)
S. dysenteriae is usually the cause of epidemics of dysentery, particularly in confined populations such as refugee camps.
Serogroup B: S. flexneri (6 serotypes)
S. flexneri is the most frequently isolated species worldwide accounts for 60% of cases in the developing world;
Serogroup C
S. boydii(23 serotypes)
Serogroup D: S. sonnei (1 serotype)
S. sonnei causes 77% of cases in the developed world
Pathogenesis of shigella infection
Shigella infection is typically via ingestion (fecal–oral contamination); depending on age and condition of the host as few as ten bacterial cells can be enough to cause an infection. causes dysentery that results in the destruction of the epithelial cells of the intestinal mucosa in the cecum and rectum.
toxins produced by shigella
Some strains produce enterotoxin & Shiga toxin, similar to the verotoxin of E. coli O157:H7. Both Shiga toxin and verotoxin are associated with causing hemolytic uremic syndrome.
The Shigella chromosomes
share most of their genes with that of E. coli K12 strain MG1655
Shigella genomes includes a virulence plasmid
Each of the Shigella genomes includes a virulence plasmid that encodes conserved primary virulence determinants.
treatment of shigella infection
Replacing fluids & electrolytes
Oral antibiotics can be given to reduce the spread in close contacts e.g.Ciprofloxacin & cephalosprins
Vaccine (live attenuated) being developed with some success against S.flexneri
Yersinia species x3
Genus contains 3 notable species
Y. enterocolitica
Y. pseudotuberculosis
Y. pestis
Y. enterocolitica
enteric pathogens acquired via consumption of contaminated food or water by animal faeces.
occurs most often in young children.
Common symptoms in children
fever, abdominal pain, and diarrhea, which is often bloody.
Symptoms typically develop 4 to 7 days after exposure and may last 1 to 3 weeks or longer.
In older children and adults,
right-sided abdominal pain and fever may be the predominant symptoms, and may be confused with appendicitis due to inflammation of mesenteric lymph nodes
Yersinia pestis
infectious agent of plague
Plague transmission
People usually get plague from being bitten by a rodent flea that is carrying the plague bacterium or by handling an infected animal.
treatment of plague
modern antibiotics are effective against plague, but if an infected person is not treated promptly, the disease is likely to cause illness or death.
Bubonic plague
painful swollen lymph nodes
bacteremia results in DIC, subcutaneous hemorrhaging & tissue death
‘Black Death’
Pneumonic plague
pulmonary distress within a day
can spread person to person via aerosols & sputum
Oxygen is essential for
A - obligate aerobes
B - obligate anaerobes
A
Why is oxygen is essential for obligate aerobes?
Serves as the final electron acceptor in electron transport chains which produce most of the ATP in these organisms.
By contrast oxygen is a deadly poison for obligate anaerobes
How can oxygen be essential for one group of organisms and yet be a fatal toxin for others?
Neither gaseous atmospheric oxygen (O2) nor the covalently bound oxygen in compounds such as carbohydrates or water is poisonous
The forms of oxygen that are toxic are those that are highly reactive.
Why are highly reactive forms of oxygen toxic?
because in the same way that oxygen is the final oxygen acceptor for aerobes, they are excellent oxidizing agents,
i.e. they steal electrons from other compounds
How toxic forms of oxygen cause a chain reaction that damages cells?
The electron depleted compounds then steal electrons from other compounds
Resulting in a chain of vigorous oxidation
Causing irreparable damage to cells by oxidising important compounds including proteins and lipids
Singlet oxygen (1O2)
toxic form of oxygen - A very reactive oxidizing agent
Molecular oxygen – electrons boosted to a higher energy state – during aerobic metabolism
Phagocytic cells – certain human white blood cells use it to oxidize pathogens
Superoxide radical (O2-)
Superoxide radicals form
- during incomplete reduction of O2 during electron transport in aerobes
- during metabolism by anaerobes in the presence of oxygen
How aerobic organisms detoxify superoxide radicals
Produce superoxide dismutase – lacking in anaerobes
Have active sites that contain metal ions e.g. Zn2+ and Fe2+
Combine 2 superoxide radicals and 2 protons to form hydrogen peroxide (H2O2) and oxygen
Peroxide anion O22-
Hydrogen peroxide produced during reactions catalysed by superoxide dismutase
Peroxide anion makes hydrogen peroxide an antimicrobial agent
What do aerobes have to detoxify the peroxidase anion?
either catalase or peroxidase
Hydroxyl radical (OH)
Hydroxyl radicals result from ionising radiation and from incomplete reduction of hydrogen peroxide
What is the most reactive of the 4?
Hydroxyl radicals
Due to catalase and peroxidase effect eliminated in aerobes
other antioxidants aerobes can use
Vitamin C & E
Again, provide electrons that reduce toxic forms of oxygen
Facultative anaerobes
Can live in various oxygen concentrations
How do facultative anaerobes maintain life
Can maintain life via fermentation or anaerobic respiration
Metabolic efficiency in absence of oxygen for facultative anaerobes
reduced
Example of facultative anaerobe
E.coli
Aerotolerant anaerobes
Do not use aerobic metabolism
Have some detoxifying enzymes
example of a Aerotolerant anaerobe
lactobacilli
Microaerophiles
Microaerophiles are damaged by the 21% concentration of atmospheric oxygen
Some organisms require oxygen levels of 2% to 10%
Example of microaerophile
Helicobacter pylori
ulcer causing pathogen
concentration of oxygen in stomach 2-10%
Injecting Drug User (IDU) Infections
40% of IDU hospital admissions due to infections, 20% result in death
Intravenous, intra-muscular or subcutaneous injection
Minor bacterial infections usually result in local abscess formation
Severe IDU infections
Severe illness if the injected material or paraphernalia, are contaminated with certain clostridial spores
Clostridia
Gram positive anaerobic spore-forming rods
Where is Clostridia found?
Widely distributed in soil and gut
How does Clostridia exist?
exo-spores
Resistant to environmental conditions
Spores germinate when introduced into an oxygen-reduced environment
Pathogenic Clostridia species
- C. perfringens*
- C. septicum*
- C. sordellii*
- C. novyi*
- C. histolyticum*
- C. tetani*
- C. botulinum*
- C. difficile*
IDU Outbreak, 2000
Cases of serious illness and deaths amongst IDUs recorded in parts of UK
60 IDUs in Scotland acquired a severe infection at or near an injection site
clinical manifestations of IDU outbreak
Spread rapidly
Extensive skin and muscle damage
Hypotension
Multi-organ failure
23 deaths
origin of IDU outbreak
Association with a batch of heroin in circulation at the time and the practice of skin or muscle “popping”
Clinical Presentation of Clostridia infection; Soft tissue inflammation at injection site
Abscess, Cellulitis, Fasciitis, Myositis
Clinical Presentation of Clostridia infection; Local inflammatory reaction has varied
Minimal pain and swelling at injection site
Clinical Presentation of Clostridia infection; Severe local symptoms
Extensive swelling, Pain, Oedema, Erythema with blackening/blistering at centre, Extensive necrosis, Necrotising fasciitis
C.novyi Type A
Widely distributed in soil
Gram-variable rods, some with sub-terminal spores
Identifying C.novyi Type A
Examine anaerobic cultures after 24h incubation for small, flat, rough or rhizoidal, translucent, haemolytic colonies with a spreading edge
Exposure to air toxic to micro-colonies that haven’t begun sporulation
After 48-72h, colonies often coalesce to give a fine spreading growth
Unreactive in commercial anaerobe identification kits (API Anaerobe)
C.perfringens
Post-mortem contaminant
Straight-sided, gram variable rods, no spores
Identifying C.perfringens morphological appearance
Large discrete colonies after 24h incubation
Flat and rough-edged, or smooth and domed
Non-haemolytic or with a narrow zone of complete haemolysis inside a larger zone of partial haemolysis
C.septicum
Gram variable rods, numerous sub-terminal spores
Most common source of isolates from blood cultures of patients with malignancies of the colon
Culture growth of C.septicum
Grows rapidly
Thick, swarming growth, haemolytic
C.botulinum
Profuse sub-terminal and free spores, gram variable bacilli
Implicated in food-borne illnesses and cases of wound botulism
Culture growth of C.botulinum
Proteolytic types A,B and F initially produce discrete rhizoidal colonies that spread and coalesce
Haemolysis is variable
C.tetani history in human disease
Uncommon in recent decades
Outbreak between July ‘03 and March ‘04, 22 cases in IDUs
Culture growth and identification of C.tetani
Colonies may produce a fine swarming growth
Gram stain of overnight cultures can give readily over-decolorised long bacilli without spores
Classical ‘drumstick’ appearance of cells with terminal, round spores after further incubation
