Gram Positive Bacteria

More than 90% of characterised genera and species come from four phyla: Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes. Actinobacteria and Firmicutes are Gram positive. Proteobacteria and Bacteroidetes are Gram negative.

Gram positive bacteria appear purple under a microscope when a Gram stain is carried out.

Gram stain procedure: Heat fixed smear flooded with crystal violet. After a minute, iodine is used to wash the sample for a further minute. Alcohol is used to decolourise any Gram negative cells. Safranin added for Gram negative stain.

Gram positive cell walls can contain up to 90% peptidoglycan. It’s common for teichoic acids embedded in their cell walls. Lipoteichoic acids are teichoic acids covalently bound to membrane lipids.

Morphology = cell shape. Major cell morphologies are coccus (spherical/ovoid), rod (cylindrical) or spirillum (spiral). There’s many variations on basic morphological types. Cells with unusual shapes include spirochetes, appendaged and filamentous bacteria.

Corynebacterium, Arthrobacter are a very large group made up of 30 taxonomic families which range from very simple to very complex. Examples are C. diptheriae and C. glutamicum, the latter which produces MSG.

Mycobacteria are rod shaped and their cell walls contain mycolic acids. An acid fast test is carried out if a person is suspected to be infected with Mycobacterium. Examples of mycobacteria are M. tuberculosis and M. leprae. Both conditions caused by these bacterias → granuloma formation. M. leprae is a very slow organism which is difficult to grow in a lab (can be grown in armadillo foot). Leprosy is a very difficult condition to treat.

Streptomyces and Norcardia are examples of filamentous bacteria. They differentiate from spore to hyphae to spore and inhabit soil. These bacteria are responsible for producing 70% of all antibiotics.

Key genera in the Mycoplasmatota bacteria group (previously named Terenicutes) are Mycoplasma and Spiroplasma. Unusually for Gram positive bacteria, these bacteria lack cell walls. They are some of the smallest organisms capable of autonomous growth and are parasites which inhabit animal and plant hosts. Mycoplasmatota lack key peptidoglycan compounds. Mycoplasma cells are pleomorphic meaning cells may be cocci or filaments of various lengths.

Key Firmicutes genera are Lactobacillus and Streptococcus, bacteria which produce lactic acid through fermentation of lactose. Lactobacillus are rod shaped and grow in chains. they are common in dairy products and also found in sauerkraut, and can grow in a pH as low as 4.

Yoghurt is a mixed culture of Streptococcus salivarius subspecies thermophilus and Lactobillus delbrueckii subspecies bulgaricus.

Streptococcus are coccus shaped and grow in chains. They play key roles in production of buttermilk, silage and other products. Some Streptococcus species are pathogenic eg Enterococcus (causes UTIs, bacteraemia, endocarditis, diverticulitis and meningitis), and Streptococcus (plaque and pathogens).

Streptococcus species = dental plaque and caries. Streptococcus viridians = scarlet fever. Streptococcus pyogenes = strep throat and necrotising fasciitis. Streptococcus pneumoniae = pneumonia.

An example of a nonsporulating Bacillales is Staphylococcus. Staphylococcus are common commensals of humans and animals. S. epidermidis is a harmless bacteria typically found on the skin.

S. aureus is associated with many pathological conditions and many strains are antibiotic resistant eg methicillin resistant S. aureus (MRSA). S. epidermidis protects us from S. aureus however when S. aureus breaks past this protective bacteria, it is one of the most common causes of sepsis. If a patient has good liver function, vancomycin can be used to treat MRSA, however some cases of vancomycin resistant MRSA have been found.

Some examples of sporulating low GC content Bacillales and Clostridiales (both Firmicutes) are Bacillus, Clostridium and Sporosarcina. They are distinguished on the basis of cell morphology and shape and cellular position of the endospore. These bacteria are generally found in soils as endospores are advantageous for soil microorganisms. Spores are resistant extreme conditions and antibiotics. B. subtilis is a Gram positive equivalent of E. coli as a model organism. Another example is B. anthracis.

B. anthracis spores in the soil may be taken up when cows consume grass. This allows the bacteria to germinate and multiply. The bacteria product toxins and stress factors, as well as spores. The spores are excreted and the cycle continues. B. anthracis isn’t found often in the UK, however it may be brought in on imported leather.

Anthrax can be cutaneous, gastrointestinal or inhalational (pulmonary). Cutaneous anthrax has the best prognosis and is developed from an insect bite or cut etc. Gastrointestinal anthrax has an intermediate prognosis and comes from eating contaminated meat. Pulmonary anthrax has a poor prognosis and comes from inhaling spores. All forms can progress to fatal systemic anthrax if untreated.

Vegetative B. anthracis cells have both a capsule and S layer which allows them to avoid being killed. The capsule is a linear polymer of γ-D-glutamic acid and the S layer is a proteinaceous paracrystalline sheath.

Clostridium lacks a respiratory chain, meaning it is anaerobic. They produce ATP via substrate level phosphorylation. Some species of Clostridium perform Stickland reactions, which is the metabolism of a pair of amino acids. Clostridium are mainly found in anaerobic pockets in the soil although they also live in the mammalian intestinal tract. Some species are pathogenic, causing diseases such as botulism, tetanus and gangrene.

Toxins must be able to cross the physical barrier of the cell membrane. AB toxins are toxins with an intracellular target and they have two functionally distinct regions, A and B. A has enzymatic activity and is responsible for all the damage caused by the toxin, however A must be associated with B as B is responsible for binding to the host cell and transporting A into the cell.

There are two different ways in which AB toxins may work. AB toxins may be a single polypeptide that’s proteolytically cleaved at a later stage or they could be separate polypeptides that subsequently assemble.

How do anthrax toxins work?

Primary antigen (PA) is released from B. anthracis cells as an 83 kDa precursor and binds ubiquitous receptors on many cell types. A 20kDa fragment is cleaved by proteases and the remaining 63 kDa fragment forms a heptamer, exposing lethal (LF) and edema factor (EF) binding sites. These factors trigger endocytosis and the acidic pH triggers translocation of EF and LF.

The LF enters cytosol and cleaves the N terminal portion of mitogen activated protein kinase kinases (MAPKKs). MAPKKs are involved in signalling pathways responsible for cytokine release, TNF and interleukin and induce cellular necrosis and toxin induced shock.

Anthrax EF remains associated with the membrane, greatly increasing cAMP conc and influencing cytokine production. May also increase host susceptibility.

Synergism between MAPKKs and cAMP increases effects of LF and EF.