Gram negative bacteria Flashcards
Where is LPS found?
Endotoxin forms the outer leaflet of the outer membrane of Gram negatives.
What does LPS comprise of?
Lipid A - toxic portion
Core (C) antigen (core oligosaccharide) - short chain of sugars, some are unique to LPS
Somatic (O) antigen (O-polysaccharide) - a highly antigenic repeating chain of oligosaccharides.
Virulence factors:
Any product or strategy that contributes to pathogenicity.
Colonisation factors: adhesins, invasins, nutrient acquisition.
Toxins: usually secreted proteins (cause damage and subversion).
Gram negative rod bacteria
Coliforms: Escherichia; Klebsiella, Salmonella, Shigella, Cirobacter, Proteus, Yersinia
Describe coliforms:
- Enterobacteria
- Rod shaped
- Motile (peritrichous flagella)
- Facultative anaerobic
- Colonise the intestinal tract
What metabolic process helps distinguish between pathogenic Enterobacteria?
Lactose fermentation - differentiate between E. coli and Klebsiella sp.
MacConkey-lactose agar
Lactose fermenters – red (pink)
Acid produced by fermentation turns neutral red dye in plate red.
Xylose Lysine Deoxycholate (XLD)
Lactose fermenters turn phenol red in media yellow
Isolates Salmonella and Shigella.
Shigella cannot ferment lactose remains red.
Salmonella cannot ferment lactose but reduce thiosulphate to produce hydrogen sulphide (black).
Further discriminate between species by…
Serology
Cell surface antigens of Gram -ve bacteria
Amino acid or carbohydrate variation in cell surface structures gives rise to antigenic variation among species AND between isolates (strains) of the same species.
Capsula - polysaccharide
LPS - polysaccharide
Flagellum - protein
Serovars examples
E. coli O157:H7 - Enterohaemorrhagic E. coli (EHEC).
E. coli O45:K1:H7 - Neonatal meningitis-associated E. coli NMEC
E. coli
Commensals
Most abundant facultative anaerobe (107-108/g faeces)
Gastroenteritis
Travellers’ diarrhoea
Bacteraemia (potentially sepsis syndrome)
Meningitis (infants) – rare in UK
Principal infections caused by pathogenic E. coli
- Wound infections (surgical)
- UTIs (Cystitis 75-80% of female UTIs –faecal source or sexual activity
Catheterisation – most common type of nosocomial infection) - Gastroenteritis
- Travellers’ diarrhoea
Bacteraemia (? sepsis syndrome)
Meningitis in infants
ETEC
Stimulates CFTR (Chlorine ion transporter) in intestinal lumen to release ions and water through toxins (heat-labile toxin and heat stable toxin) – leads to watery diarrhoea.
EPEC
Invades cells in small intestine.
EIEC
Enteroinvasive Escherichia coli
Bacteria
Invades cells in large intestine.
Why pathogenic?
Acquisition of pathogenicity genes ‘en bloc’ by ‘lateral gene transfer’.
What are the four species of Shigella?
S. dysenteriae
S. flexneri
S. boydii
S. sonnei
Shigellosis
Shigellosis: severe bloody diarrhoea (bacillary dysentery)
S. dysenteriae causes most severe form
S. sonnei most prevalent in developed world
Symptoms of shigellosis
Frequent passage of stools (>30/day)
Small volume, pus and blood, prostrating cramps, pain in straining and fever.
Pathogenesis of Shigella infection
- Acid tolerant with a low infective dose
- Person to person, or contaminated water and food (fresh raw vegetables used in salad)
- Entry through colonic M cells (antigen sampling cells, overlie lymphoid follicles and deliver AGs to underlying immune cells.
- Induced uptake
- Shiga toxin inhibits protein synthesis -> cell death.
- Systemic absorption of Shiga toxin - targets kidneys - haemolytic uraemic syndrome / microvascular thrombosis in kidneys - kidney failure.
What are the three forms of salmonellosis caused by S. enterica?
- Gastroenteritis/enterocolitis (serovars Enteritidis and Typhimurium)
Frequent cause of food poisoning (milk, poultry meat & eggs)
Second highest no. of food-related hospitalisations/deaths (UK)
6-36 hr incubation period, resolves (~7 days)
Localised infection, only occasionally systemic - Enteric fever - typhoid/paratyphoid fever (serovars Typhi and Paratyphi)
Poor quality drinking water/poor sanitation
Systemic disease
~20 million cases, ~200,000 deaths/year (globally) - Bacteraemia (serovars Cholerasuis and Dublin)
Uncommon
Pathogenesis of slamonellosis
Ingestion of contaminated food/water - high I.D.
- invasion of gut epithelium
- transcytosed to basolateral membrane
- enters submucosal macrophages
- intracellular survival/replication
Serovars Enteritidis and Typhimurium
Localised intestinal secretory and inflammatory response.
Do not produce toxins.
Serovar Typhi
Systemic infection due to dissemination within macrophages.
Produces typhoid toxin (DNA-ase activity = genotoxin).
Proteus mirabilis (pathogenic enterobacteria)
Can differentiate into an elongated hyperflagellated form = surface motility (‘swarming’)
Causes catheter-associated UTIs (~30% cases) pyelonephritis
Produces urease (causes urine pH increase) - calcium phosphate precipitation - formation of bladder/kidney stones, catheter blockage.
Klebsiella pneumoniae (pathogenic enterobacteria)
Environmental.
Opportunistic, nosocomial infections (neonates, elderly, compromised)
Colonisation of gastrointestinal tract (normal) and oropharynx (less frequently) is benign but can lead to:
-UTI
-pneumonia (aspiration from oropharynx)
-surgical wound infections
-bacteraemia = sepsis (high mortality)
Multi-drug resistant (resistant to carbapenems)
Pseudomonas aeruginosa
Ubiquitous, free-living
Motile (single polar flagellum)
Rod-shaped
Opportunistic (serious cause of nosocomial infections)
Resistant to multiple antibiotics (& disinfectants) - very difficult to treat
Acute infections (due to multiple toxins):
Localised
Burn/surgical wounds, UTI, Keratitis
Systemic (bacteraemic = sepsis)
neutropenic patients (leukaemia, chemotherapy, AIDS)
ICU patients (ventilator acquired pneumonia)
leading cause of nosocomial pneumonia
Chronic infections:
Cystic fibrosis (CF) patients
Common denominator to all infections - compromised host defences.
P. aeruginosa and CF
CF, most common inherited lethal disorder among Caucasians.
Defective CFTR = thick mucus produced in lungs.
Lungs prone to microbial infection.
P. aeruginosa most problematic.
P. aeruginosa ‘wild-type’ (non-mucoid).
P. aeruginosa ‘CF’ phenotype (mucoid)
Isolates that secrete a thick coating of exopolysaccharide: provides additional protection against host defences in the lung
Chronic inflammation leading to progressive lung damage and deterioration of lung function
Occurs following infection of the CF lung by P. aeruginosa
Mainly due to host immune system trying to clear the infection.
Vibrio cholerae
Facultative anaerobe
Saline environments: commensal to planktonic crustaceans such as copepods
Ingestion by shellfish
Contamination of drinking water due to flooding of coastal areas or poor sanitation (faecal contamination).
Cholera
Most severe diarrhoeal disease
Characterised by pandemics (7 recorded since 1817)
1P-6P, Indian subcontinent
7P began in Sulawesi (1961) SE Asia (1963) Africa (1970) Latin America (1991) Caribbean (2010)
O1 serotype epidemics (and occasionally O139 variant)
1.4-4.0 million cases/year, 20,000-140,000 associated deaths
Pathogenesis of V. cholerae
Faecal-oral route (not person-to-person) - high infective dose required
Faecal contaminated water (poor sanitation)
Undercooked shellfish from risk areas)
Incubation, few hours to 5 days (V.c. multiplies in small intestine)
Voluminous watery stools (‘secretory’ diarrhoea)
Can lose 20 litres fluid/day plus electrolytes
Dehydration/death (hypovolaemic shock)
50-60% mortality if untreated
No blood, pus or fever (i.e. not dysenteric)
i.e. no invasion or damage to mucosa
Release of cholera toxin causes pathogenesis
Most cases can be treated with ORT
Campylobacter
C. jejuni
C. coli
Spiral rods
Unipolar (monotrichous) or bipolar (amphitrichous) flagella
Most common cause of food poisoning in UK & US
Undercooked poultry
Cattle (unpasteurised milk)
I.D. 500-800 (low)
Mild to severe diarrhoea, often with blood
Usually self-limiting (≤ 1 week)
Campylobacter shed in faeces for ~3 weeks
Helicobacter pylori
Spiral shaped
Tuft of polar flagella
Discovered in gastric mucus, 1982 (stomach previously thought to be sterile)
Present in ~50% global population, but only a fraction develop disease
Major role in gastritis and peptic ulcer disease (80-90% of ulcers)
Implicated in ~10% cases of gastric adenocarcinoma & mucosa-associated lymphoid tissue lymphoma (linked to production of VacA and CagA toxins)
Barry Marshall ingested H. pylori → gastritis → Nobel prize 2005
Haemophilus influenzae
Exclusively human reservoir
Nasopharyngeal carriage in 25-80% population (non-capsulate strains)
Transient carriage of capsulate strains occurs in 5-10%
What are the opportunistic infections caused by H. influenzae?
Opportunistic infections seen mainly in young children and adult smokers:
- Meningitis* (age <5 yrs), 5-10% of adult cases
- Bronchopneumonia
- Epiglottitis*, sinusitis, otitis media
- Bacteraemia* (often associated with pharyngitis)
- Pneumonia in CF, COPD, HIV patients
*infections caused by capsulate strains (invasive) – uncommon in healthy adults
Diagnostics for H. influenzae
Fastidious
Requires ‘factor X’ (haem) and ‘factor Y’ (NAD)
Cultured on chocolate agar
Non-motile.
What are the virulence determinants of H. influenzae?
Capsule - invasive strains are capsulate (‘encapsulated’)
Can penetrate nasopharyngeal epithelium
Resistance to phagocytosis and complement system
6 different capsule serotypes (a-f)
Hi b strains are the main cause of meningitis
‘Hib’ vaccine has reduced the incidence
Commensals and upper respiratory tract pathogens are non-capsulate (‘unencapsulated’) referred to as ‘non-typeable’ H. influenzae (NTHi)
Incidence increasing
LPS (‘endotoxin’)
Inflammation
Complement resistance
Bordetella pertussis
Short (sometimes oval) rods (‘coccobacilli’)
Fastidious
Humans - only known reservoir (obligate human pathogen)
Pertussis (whooping cough)
B. parapertussis causes mild pharyngitis
Highly contagious (low I.D.) - aerosol transmission
Non-specific flu-like symptoms (~7 d), followed by paroxysmal coughing
https://www.youtube.com/watch?v=l5SHtdczSBc
Non-invasive
Produces two toxins
Pertussis toxin
CyaA
Hypersynthesis of cAMP suppresses innate immune function particularly macrophage phagocytosis
Legionella pneumophila
Discovered 1976
Legionnaires’ disease - severe inflammatory pneumonia (1-3% of all pneumonias)
Immunocompromised (elderly, alcoholics, smokers)
Severe (15-20% mortality)
Infection from man-made aquatic environments
air-conditioning cooling towers, shower heads, nebulisers, humidifiers…
Replicate within freshwater protozoa - intracellular parasite of amoeba
Can survive and replicate within alveolar macrophages
Upregulates pro-inflammatory genes in alveolar macrophages
Excessive influx of neutrophils into lungs → inflammation.
Bacteroides
Non-motile rods
Strict (obligate) anaerobes
Commensal flora (large intestine) - most abundant (30-40% of the total)
>1010/g faeces (outnumbers E. coli 20:1)
Opportunistic - tissue injury (surgery, perforated appendix or ulcer)
Predominantly peritoneal cavity infections (peritonitis, intraabdominal abscesses are most common) can lead to bacteraemia
Most frequent cause of anaerobic infections, usually B. fragilis (although it is only 0.5-1.0% of total commensal Bacteroides)
Often present in polymicrobial infections with enterobacteria
Presence of facultative anaerobes depletes O2, allowing anaerobes such as Bacteroides to proliferate
Treatment requires specific anti-anaerobe antibiotics e.g. metronidazole
Neisseria (growing as single cells)
Non-flagellated diplococci
Fastidious
Two species of medical importance:
N. meningitidis
N. gonorrhoeae
Humans are the only known reservoir.
Neisseria meningitidis
Meningococcus
Nasopharyngeal carriage in 5-10% population (asymptomatic)
Rises to 20-90% during outbreaks
Person-to-person (aerosol) transmission (universities, barracks, Haj)
Pathogenesis:
Crosses nasopharyngeal epithelium and enters bloodstream
Low level bacteraemia (asymptomatic) or septicaemia (sepsis)
Meningitis: invasion of the meninges - bacteria enter CSF of subarachnoid space
second most frequent cause of meningitis in young children
Very high mortality from septicaemia if not treated
Requires rapid diagnosis!
Virulence determinants of N. meningitidis
Capsule is major virulence determinant (serogroup B - 90% cases in UK)
Anti-phagocytic
Noncapsulated N.m. only found in nasopharynx - not pathogenic
LPS (membrane ‘blebs’)
Cytokine cascade
Sepsis
N. gonorrhoeae
Gonococcus
Gonorrhoea - second most common STD worldwide
82.4 million (2020)
Cases doubled in the UK last year
Person-to-person only
Infection can be asymptomatic (~10% men, ~50% women)
Usually characterised by urethritis with additional infection of female genitalia
Serious complications in women - can lead to salpingitis and/or PID if infection ascends.
Proctitis, gingivitis, pharyngitis depending on sexual preference
Multi-drug resistance arising
Note: gonococci are non-capsulated (unlike pathogenic N. meningitidis)
What bacteria grow as single cells and belong to spirochaetes?
Leptospira
Treponema
Borrelia
Spirochaetes
Long, slender, helical, highly flexible
Most are free-living and non-pathogenic
Pathogenic varieties difficult to culture
Modified outer membrane (“outer sheath”)
Treponema and Borrelia lack LPS, replaced by a different glycolipid
Endoflagella (‘axial filaments’ = periplasmic flagella)
Located between peptidoglycan and outer membrane
Fixed at each end of the bacterium and confers shape
Overlap in the centre of the bacterium.
Propels bacterium in a corkscrew motion
Swim faster in high viscosity medium
“Hides” antigenic flagellum
Three medically important genera
Borrelia burgdorferi
Lyme disease (zoonosis) (~300 cases in UK)
B.b. infects small mammals (rodents)
B.b. acquired by tick larvae feeding on infected animal
Transmitted to humans by tick nymphs (adults easier to spot!)
bull’s eye rash, flu-like symptoms (fever, fatigue, headache)
Dissemination via lymphatics/blood to other organs
neurological problems in 10-15% patients, joints → arthritis
Most symptoms arise due to immune response
Leptospira interrogans
Leptospirosis (zoonosis)
Rare in UK
Common infection of rats
Systemic infection of the rat, excreted in faeces and urine
Proximity of rats to water often means water sources are colonised
Rivers, streams country parks, lakes
Contact of infected animal urine with mucous membrane or abraded skin
Flu-like symptoms
Severe form (Weil’s disease*) in 10-15% infected individuals
2-4 week incubation period, clinical multi-organ infection occurs
Liver infection leads to cell destruction and jaundice
Acute renal and hepatic failure
Pulmonary distress
Haemorrhage
Treponema pallidum
Syphilis (STD)
2,800 cases in UK – increasing again
Primary stage
Localised genital infection (ulcer (“chancre”))
Days-weeks post-infection
Highly transmissible phase
Secondary stage (~50% cases)
Systemic
Skin (rash), swollen lymph nodes, joint pains, muscle aches, headache, fever
1-3 months post-infection
Still highly transmissible
Tertiary stage (~30% cases)
‘Gummas’ (granulomas) in bone and soft tissue
Cardiovascular syphilis (aorta)
Neurosyphilis (brain and spinal cord)
Occurs several years post-infection
Non-infectious form
All stages are treatable with ABx but treatment is longer for tertiary stage syphilis and tertiary damage cannot be reverted.
Obligate intracellular bacteria
Rickettsia
Chlamydia
Coxiella
Chlamydia
Very small, non-motile
Obligate intracellular parasites
Many species within this group live asymptomatically as endosymbionts in amoebae, invertebrates and vertebrates
Cannot culture in bacteriological media - detect by serum Abs or PCR
Detects presence of anti-Chlamydia antibodies in serum
May be the result of a previous infection
Test must not be used to detect a current infection without an alternative confirmatory test
Also referred to as NAA (nucleic acid amplification) or NAAT (nucleic acid amplification test)
Life cycle of Chlamydia
Unique growth cycle: 2 developmental stages:
Elementary bodies (EBs)
Rigid, extracellular form, ~0.3 μm, dormant
Infectious
Enter cell through endocytosis
Prevent phagosome-lysosome fusion
↓ differentiates into...
Reticulate bodies (RBs)
Fragile, intracellular form, ~1.0 μm, metabolically active
Replicative
Non-infectious
Acquire nutrients from host cell
Medically important members of Chalmydia
C. trachomatis (3 biovars):
Trachoma biovar (serotypes A-C) → trachoma → blindness
Eye-to-eye transmission via hands, fomites or flies
Genital tract biovar (serotypes D-K)
most common STD - infects epithelial cells of mucous membranes of urethra (both sexes) and vagina
can ascend to uterus and ovaries (PID, infertility)
usually asymptomatic (i.e. 70-80% cases in women)
conjunctivitis (STD), hand-to-eye transmission
Lympho granuloma venereum (LGV) biovar (serotypes L1-L3)
causes LGV (an STD) - invasive urogenital or anorectal infection
endemic to the tropics, cases rising in Europe/N. America
C. pneumoniae
Respiratory tract (mild or “walking” pneumonia)
~10% community acquired pneumonias
C. psittaci
Mainly birds
Psittacosis (zoonotic infection), severe pneumonia
