Gram-negative bacteria

The outer membrane is the defining feature of Gram negative bacteria as they have a much thinner peptidoglycan layer.

Gram-negative bacteria may have fimbriae for adhesion and/or flagella for motility. Capsule production is more common in Gram negative bacteria. Conjugative pilli facilitate transfer DNA transfer between bacteria during conjugation. The only feature specific to Gram negative bacteria is the type III secretion system which is a key virulence factor.

The inner and outer leaflet of the Gram negative bacteria cell envelope are completely different in composition. The outer leaflet mainly consists of lipopolysaccharides and the inner leaflet mainly consists of phospholipids.

The Gram-negative outer membrane is the first line of defence against antibiotic therapy, disinfectants and the host immune response. It acts as a selective barrier. Nutrients and small antibiotics enter through protein channels and large hydrophilic antibiotics are blocked. The membrane protects cells from bile salts eg cholic acid, deoxycholic acid and other detergents eg SDS. Proteins contained within the membrane layers are β-barrel outer membrane proteins and lipoproteins.

There were over 1 million deaths due to antibiotic resistant bacterial infections in 2019. Antibiotics are often ineffective against Gram negative bacteria due to lack of permeability, and any antibiotics which can pass through the membrane have a slow rate of diffusion and are often removed via efflux pumps. Gram negative bacteria E.coli, K. pneumoniae and P. aeruginosa account for 39% of global bloodstream infections. The healthcare costs associated with hospital acquired antibiotic resistant Gram-negative infections are very high.

Many Gram negative bacteria are a major component of the human gut microbiota.

Proteobacteria are found in all major ecosystems but they aren’t a major component of the normal human gut microbiome. Elevated levels of Proteobacteria in the gut usually indicates a dysbiotic microbiome eg increased E. coli levels in Crohn’s.

Proteobacteria includes many of the most commonly encountered bacteria and are the most metabolically diverse of all bacteria, including chemolithotrophs, chemoorganotrophs and phototrophs. They are morphologically diverse and divided into alpha-, beta-, delta-, gamma- and epsilon-.

There are nearly 1000 Alphaproteobacteria species. Many are obligate or facultative aerobes. Many oligotrophic. The six major orders include Rhizobiales, Rickettsiales, Rhodobacterales, Rhodospirallales, Caulobacterales, and Sphingomonadales.

Rickettsia (order of Alphaproteobacteria) species are obligate intracellular bacteria mainly associated with arthropods. Small coccoid or rod shaped. They have a reduced genome so are unable to synthesise certain metabolites, meaning they must obtain them from the host. Some species are able to infect mammalian hosts, mainly through arthropod bites or faeces eg Rickettsia prowazekii → epidemic typhus and Rickettsia ricketsii → Rocky Mountain spotted fever.

Caulobacter crescentus is a model for asymmetric division and cellular differentiation as they produce very distinct stalked and swarmer cells. The stalked cells form biofilms to exploit nutrient sources and the swarmer cells are motile to seek out new environments.

There are around 500 described Betaproteobacteria and they display a lot of functional diversity. The six major orders are 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) are purple non-sulphur bacteria which grow best as photoheterotrophs but can also grow as photoautotrophs and by respiration.

Zooglea (genera of Rhodocyclales) are chemoorganotrophs which produce a thick capsule and are important in wastewater treatment.

Neisseria meningitidis causes meningitis and meningococcal septicaemia which are severe and often fatal. Both of these conditions are most common in infants, teens and young adults. Around 10% of adults carry Neisseria meningitidis in their nasopharynx without harm. Neisseria meningitidis exclusively colonises and infects humans as it can only acquire iron from human transferrin and lactoferrin.

Neisseria gonorrhoeae → gonorrhoea. 1/10 cases asymptomatic and highly drug-resistant strains are spreading.

Bordatella pertussis → pertussis aka whooping cough. Pertussis is a severe and highly infectious respiratory disease which mostly affects children. Prevented by vaccination, treated with antibiotics.

A key order of the class Gammaproteobacteria is Enterobacteriales. Key Enterobacteriales genera are Enterobacter, Escherichia, Klebsiella, Proteus, Salmonella, Serratia and Shigella. Enteric bacteria are facultative aerobic nonsporulating rod shaped bacteria which may be motile or non-motile. They have relatively simple nutritional needs and are able to ferment sugars to a variety of end products. Enteric bacteria can be separated into two broad groups by type and proportion of fermentation products generated by anaerobic fermentation of glucose. They may be mixed-acid fermenters or 2,3-butanediol fermenters.

Examples of mixed acid fermenters are Escherichia, Salmonella, Shigella, Citrobacter, Proteus and Yersinia.

Escherichia are universal inhabitants of warm blooded animals’ intestinal tract and they synthesise vitamins K and B12. They’re peritrichously flagellated or non-motile. Presence in water is indicative of recent faecal contamination.

Salmonella and Shigella are usually pathogenic. Salmonella characterised by surface antigen. The O antigen is based on the outer region of the lipopolysaccharide and the H antigen is based on flagellar proteins.

Key virulence factors of pathogenic E.coli are often encoded by mobile genetic elements. E.coli’s applications in biotechnology include recombinant proteins, biofuels, synthetic biology and more. E.coli can be grown at 37 degrees Celsius, has a doubling time of 20 minutes and is metabolically diverse.

Butanediol fermenters are a closely related group of organisms, with example genera being Enterobacter, Klebsiella, Erwinia and Serratia. Enterobacter are found in water, sewage and the intestinal tract of warm blooded animals. They may cause UTIs.

Klebsiella are found in soil and water. Most strains fix nitrogen. Klebsiella pneumoniae causes severe hospital acquired infections including pneumoniae, UTIs and bacteraemia. Resistance to last resort antibiotics eg carbapenems is becoming a major issue.

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

Pseudomonadales are chemoorganotrophs which use a wide variety of organic compounds. They’re oxidase and catalase positive and can cause plant and animal diseases.

Key genera of epsilonproteobacteria are campylobacter, helicobacter and sulphurospirillum. Campylobacter and Helicobacter are pathogenic gram negative, motile spirilla which are oxidase and catalase positive. Sulphurospirillum are free-living microaerophils which anaerobically respire.

Campylobacter jejuni causes foodborne illness, most commonly from raw shellfish or undercooked poultry/pork. Can be passed person to person if hygiene poor. Incubation period 1 to 11 days, although usually between 2 and 5. Symptoms include abdominal pain, diarrhoea and malaise. Relatively low infective dose.

Key Deltaproteobacteria genera are Bdellovibrio, Myxococcus, Desulfovibrio, Geobacter and Syntrophobacter. These bacteria are sulphate and sulphur reducers, dissimilative iron reducers and bacterial predators.

Following contact with a gram negative bacteria, the highly motile Bdellovibrio cell attaches to and penetrates the prey periplasmic space. Once inside, Bdellovibrio elongates, releasing progeny cells within 4 hours. Number of progeny cells released varies with size of prey eg 5 to 6 Bdellovibrios are released from E. coli.

Myxobacteria have the most complex behavioural patterns of all known bacteria. Their life cycle results in the formation of multicellular structures known as fruiting bodies. They form a swarm to glide over surfaces and obtain their nutrients by using extracellular enzymes to lyse other bacteria.

Life cycle of Myxococcus xanthus: Germination of myxospores is chemically induced. Germination causes vegetative cells to outgrow the spores. The vegetative cycle is chemically induced. Swarming and aggregation of M. xanthus occurs creating a mound of cells which form fruiting bodies and form myxospores.