gram-negative bacteria Flashcards
what are the main classifications (shapes) of gram-negative bacteria which we look at in clinic?
- rods
- cocci
- spirochaetes
- obligate intracellular bacteria
what are the gram-negative rod family group names?
anearobic:
- bacteroridies
aerobic:
- choliform / enterobacteria
- pseudomonas
- vibro
- parvobacteria
what are the gram negative cocci groups?
anaerobic:
- veillonella
aerobic:
- neisseria
what are the gram-negative spirochates groups?
- leptospira
- treponema
- borrelia
what are the gram-negative obligate intracellular bacteria that we consider?
- rickettsia
- chlamydia
- coxiella
outline the characteristics of coliforms / enterobacteria
Enterobacteriaceae or Enterobacteria
Rod-shaped
Motile (most)
Peritrichous flagella
Facultatively anaerobic - both aerobic and aerobic
Colonise the intestinal tract
Advantageously or disadvantageously
what tests do we use to distinguish what species the gram-negative bacillus bacteria is?
assess phenotypes of different agar:
MacConkey-lactose agar
- lactose fementers turn red eg e.coli, non-lactose fermenters stay yellow eg salmonella
- it turns red because the acid produced by fermentation turns natural red dye in plate red
xylose lysine deoxycholate
- lactose fermenters turn phenol red in media yellow
- isolated salmonella and shingella
- shingella remaind red
- salmonella cannot ferment lactose but redue thiosulphate to produce hydrogen sulphide so appears black
further discrimination between species by serology based on virulence factors
antigen variation
Escherichia coli
Principal infections:
- 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 (potentially sepsis syndrome)
- Meningitis (infants) – rare in UK
Shigella
Four species:
S. dysenteriae - most severe
S. flexneri
S. boydii
S. sonnei - most prevelant
- Person-to-person
- contaminated water & food
Shigellosis: severe bloody diarrhoea (bacillary dysentery)
Symptoms and pathology:
- Frequent stools (>30/day) - small volume, pus and blood, prostrating cramps, fever
- Self-limiting (in adults)
Shiga toxin inhibits protein synthesis
Targets kidneys
Also occurs in Shiga-toxin producing EHEC
Salmonella
Two species:
S. enterica - responsible for salmonellosis
>2,500 serovars
S. bongori - rare (contact with reptiles)
Infections caused by S. enterica (salmonella)
Ingestion of contaminated food/water - high I.D. (~106) (‘faecal-oral route’)
Three forms of salmonellosis caused by S. enterica:
1. Gastroenteritis/enterocolitis (serovars Enteritidis and Typhimurium)
- 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, inflammation and necrosis of gut mucosa
- Only occasionally systemic
- Does not produce toxins
- Enteric fever - typhoid/paratyphoid fever (serovars Typhi and Paratyphi)
Poor quality drinking water/poor sanitation
Systemic disease – dissemination of macrophages
~20 million cases, ~200,000 deaths/year (globally)
Produces typhoid toxin (DNase activity = a genotoxin) - Bacteraemia (serovars Cholerasuis and Dublin)
Uncommon
Other pathogenic Enterobacteria
Proteus mirabilis
- catheter-associated UTIs (~30% cases)
- pyelonephritis
- Produces urease - inc Ca - formation of bladder/kidney stones, catheter blockage
Klebsiella pneumoniae
Environmental
Opportunistic, nosocomial infections (neonates, elderly, compromised)
Colonisation of gastrointestinal tract and oropharynx is benign but can lead to:
UTI
Pneumonia (aspiration from oropharynx)
Surgical wound infections
Bacteraemia sepsis (high mortality)
Multi-drug resistant (resistant to carbapenems)
name the species of gram-negative - rod - aerobic - choliform.
- salmonella
- shigella
- E.coli
name the species of gram-negative - rod - aerobic - pseudomonads.
Pseudomonas aeruginosa
Pseudomonas aeruginosa
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
Systemic (bacteraemic sepsis)
neutropenic patients (leukaemia, chemotherapy, AIDS)
ICU patients - nosocomial pneumonia
Cystic fibrosis (CF) patients
P. aeruginosa and cystic fibrosis
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.
name the species of gram-negative - rod - aerobic - vibro
- vibrio cholerae
- Campylobacter
- Helicobacter pylori
Vibrio cholerae
Facultative anaerobe
Saline environments: commensal to planktonic crustaceans
Ingestion by shellfish
Contamination of drinking water due to flooding or poor sanitation (faecal contamination) – high infective dose
Incubation, few hours to 5 days (multiplies in small intestine)
Cholera
Most severe diarrhoeal disease
Characterised by pandemics (7 recorded since 1817)
Most in Indian subcontinent
1.4-4.0 million cases/year, 20,000-140,000 associated deaths
Voluminous watery stools (‘secretory’ diarrhoea)
Can lose 20 litres fluid/day plus electrolytes
Dehydration/death (hypovolaemic shock) 50-60% mortality
Non-invasive no blood, pus or fever (i.e. not dysenteric)
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)
Infectious dose 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)
Oncogenic microbe - ~10% cases of gastric adenocarcinoma & mucosa-associated lymphoid tissue lymphoma
Barry Marshall ingested H. pylori → gastritis → Nobel prize 2005
name the species in the gram-negative - rod - aerobic - parvobacteria
- Haemophilus influenzae
- Bordetella pertussis
- Legionella pneumophila
Haemophilus influenzae
Exclusively human reservoir
Nasopharyngeal carriage in 25-80% population (non-capsulate strains)
Transient carriage of capsulate strains occurs in 5-10%
Opportunistic infections seen mainly in young children and smokers:
Capsulated – 6 different capsule types (a-f) – rare in adults
Meningitis (age <5 yrs), 5-10% of adult cases Hi b vaccine reduced incidence
Bacteraemia (often associated with pharyngitis)
Epiglottitis
Non-capsulated Non-typeable H. influenzae (NTHi) – incidence increasing
Bronchopneumonia
Sinusitis, otitis media
Pneumonia in CF, COPD, HIV patients
Diagnostics
Fastidious
Requires ‘factor X’ (haem) and ‘factor Y’ (NAD)
Cultured on chocolate agar
Non-motile
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
Non-invasive
Produces two major toxins
Pertussis toxin
CyaA
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
what species are involved in gram-negative - rod - anaerobic?
bacteriodes
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
name the gram-negative - cocci - aerobic.
- Neisseria
- Neisseria meningitidis
- Neisseria gonorrhoeae
Neisseria gonorrhoeae
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
Non-capsulated
Neisseria meningitidis
Nasopharyngeal carriage in 5-10% population (asymptomatic)
Person-to-person (aerosol) transmission (universities)
Pathogenesis:
Low level bacteraemia (asymptomatic) or septicaemia (sepsis)
Meningitis: invasion of the meninges - bacteria enter CSF of subarachnoid space
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)
LPS (membrane ‘blebs’)
Cytokine cascade
Sepsis
Neisseria
Non-flagellated diplococci
Fastidious – humans only known reservoir
Two species of medical importance:
N. meningitidis
N. gonorrhoeae
name the gram-negative - spirochates.
- Borrelia burgdorferi
- Leptospira interrogans
- Treponema pallidum
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)
Infects small mammals (rodents)
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
gram-negative - obligate intracellular bacteria which are clinically relevant
Chlamydia
Chlamydia
Very small, non-motile
Obligate intracellular parasites
Many species 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)
Medically important members of Chlamydia
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 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
