Gram-negative bacteria

Gram-negative bacteria

Bacteria with a thin peptidoglycan layer, type III secretion system, and which more commonly produce a capsule.

Type III secretion system

A key virulence factor specific to Gram-negative bacteria.

What makes up the majority of the outer leaflet of Gram-negative cell envelopes?

lipopolysaccharides

What is the main component of Gram negative cell envelope inner membrane

phospholipids

Gram negative outer membrane as a selective barrier

Allows entry of nutrients and small antibiotics but prevents entry of large hydrophilic antibiotics and protects from bile salts and host immune response

proteins contained within membrane layers

β-barrel outer membrane proteins and lipoproteins

Why are antibiotics often ineffective against Gram negative bacteria?

due to lack of membrane permeability. Any antibiotics that pass through the membrane diffuse slowly and are often removed via efflux pumps.

Proteobacteria

Metabolically diverse group including Alpha-, Beta-, Delta-, Gamma-, and Epsilonproteobacteria. In all major ecosystems but not a major component of the normal gut microbiome.

six major Alphaproteobacteria orders

Rhizobiales, Rickettsiales, Rhodobacterales, Rhodospirallales, Caulobacterales, and Sphingomonadales.

Rickettsia

small coccoid/rod obligate intracellular bacteria with reduced genomes. Associated with arthropods

Caulobacter crescentus

model for asymmetric division and cellular differentiation as they produce very distinct stalked and swarmer cells.

stalked cells

form biofilms to exploit nutrient sources

swarmer cells

motile to seek out new environments

six major Betaproteobacteria orders

Burkholderiales, Hydrogenophiales, Methylophilales, Neisseriales, Nitrosomonadales and Rhodocyclales

Burkholderia

grow aerobically although some can also grow anaerobically. Some species are able to fix nitrogen

Rhodocyclus (genera of Rhodocyclales)

purple non-sulphur bacteria. Grow best as photoheterotrophs. Can also grow as photoautotrophs and by respiration

Zooglea (genera of Rhodocyclales)

chemoorganotrophs which produce a thick capsule and are important in wastewater treatment

Why does Neisseria meningitidis (meningitis and meningococcal septicaemia) exclusively colonise humans?

can only acquire iron from human transferrin and lactoferrin

Neisseria gonorrhoeae

1/10 cases asymptomatic and highly drug-resistant strains are spreading

Pertussis aka whooping cough

severe, highly infectious respiratory disease caused by Bordatella pertussis. Prevented by vaccination and treated with antibiotics

key Enterobacteriales (order of Gammaproteobacteria) genera

Enterobacter, Escherichia, Klebsiella, Proteus, Salmonella, Serratia and Shigella

Enteric bacteria

facultative aerobic nonsporulating rods. May be motile or non-motile. Relatively simple nutritional needs and ferment sugars to a variety of end products

mixed acid fermenters

Escherichia, Salmonella, Shigella, Citrobacter, Proteus and Yersinia.

Escherichia

synthesise vitamins K and B12. Peritrichously flagellated or non-motile

characterisation of salmonella by surface antigen

O antigen based on the outer region of the lipopolysaccharide and H antigen based on flagellar proteins.

E.coli in biotechnology

recombinant proteins, biofuels, synthetic biology and more

Butanediol fermenters

closely related group of organisms, with example genera being Enterobacter, Klebsiella, Erwinia and Serratia.

Pseudomonas aeruginosa

causes a wide variety of infections in predisposed individuals. Major cause of serious hospital acquired infections, associated with AMR and mortality. P. aeruginosa major cause of mortality in people with cystic fibrosis.

Pseudomonadales

Oxidase and catalase positive chemoorganotrophs. Can cause plant and animal disease.

Key epsilonproteobacteria genera

campylobacter, helicobacter and sulphurospirillum

Campylobacter and Helicobacter

pathogenic motile spirilla which are oxidase and catalase positive.

Sulphurospillum

free-living microaerophiles which anaerobically respire.

Campylobacter jejuni

causes foodborne illness, most commonly from raw shellfish or undercooked poultry/pork. Incubation period 1 to 11 days, although usually between 2 and 5.

C. jejuni symptoms

abdominal pain, diarrhoea and malaise

Key Deltaproteobacteria genera

Bdellovibrio, Myxococcus, Desulfovibrio, Geobacter and Syntrophobacter. Sulphate and sulphur reducers, dissimilative iron reducers and bacterial predators.

Bdellovibrio

Highly motile Deltaproteobacteria that prey on other bacteria by penetrating their periplasmic space.

Myxobacteria

have complex behavioural patterns forming multicellular fruiting bodies and obtaining nutrients by lysing other bacteria using extracellular enzymes