:characteristics

:Characteristics 1. 2.

3. 4.

cytoplasm lipid membrane thick peptidoglycan layer o teichoic acids and lipoids are present, forming lipoteichoic acids which serve to act as chelating agents, and also for certain types of adherence. capsule polysaccharides (only in some species) flagellum (only in some species) o if present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.

:Classification In the original bacterial phyla, the Gram-positive organisms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Staphylococcus, Streptococcus, Enterococcus, (which are cocci) and Bacillus, Corynebacterium, Nocardia, Clostridium, Actinobacteria, and Listeria (which are rods and can be remembered by the mnemonic obconical). It has also been expanded to include the Mollicutes, bacteria like Mycoplasma that lack cell walls and cannot be Gram stained, but are derived from such forms. Actinobacteria are the other major group of Gram-positive bacteria, which have a high guanosine and cytosine content in their genomes (high G+C group). This contrasts with the Firmicutes, which have a low G+C content. Both Gram-positive and Gram-negative bacteria may have a membrane called an S-layer. In Gram-negative bacteria, the Slayer is directly attached to the outer membrane. In Gram-

positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.

:Exception The Deinococcus-Thermus bacteria have Gram-positive stains, although they are structurally similar to Gram-negative bacteria.

Pathogenesis: Most pathogenic bacteria in humans are gram-negative organisms. Classically, six gram-positive genera are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (round bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium and Listeria (a coccobacillus), while Bacillus and Clostridium produce spores.[3] The sporeforming bacteria can again be divided based on their respiration: Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe.

:Characteristics 1.

Cytoplasmic membrane 2. Thin peptidoglycan layer (which is much thicker in Gram-positive bacteria) 3. Outer membrane containing lip polysaccharide (LPS, which consists of lipid A, core polysaccharide, and O antigen) outside the peptidoglycan layer 4. Porins exist in the outer membrane, which act like pores for particular molecules

There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space 6. The S-layer is directly attached to the outer membrane, rather than the peptidoglycan 7. If present, flagella have four supporting rings instead of two 8. No teichoic acids or lipoteichoic acids are present 9. Lipoproteins are attached to the polysaccharide backbone. 10. Most do not sporulate (Coxiella burnetii, which produces spore-like structures, is a notable exception) 5.

Medical treatment: One of the several unique characteristics of Gram-negative bacteria is the structure of the outer membrane. The outer leaflet of the membrane comprises a complex lip polysaccharide whose lipid portion acts as an endotoxin. If endotoxin enters the circulatory system it causes a toxic reaction with the sufferer having a high temperature and respiration rate and a low blood pressure. This may lead to endotoxic shock, which may be fatal. This outer membrane protects the bacteria from several antibiotics, dyes, and detergents which would normally damage the inner membrane or cell wall (peptidoglycan). The outer membrane provides these bacteria with resistance to lysozyme and penicillin. Fortunately, alternative medicinal treatments such as lysozyme with EDTA, and the antibiotic ampicillin have been developed to combat the protective outer membrane of some pathogenic Gram-negative organisms. Other drugs can be used, namely chloramphenicol, streptomycin, and nalidixic acid.

:Example species Proteobacteria are a major group ofGram-negative bacteria,including: Escherichia col,and other Enterobacteriaceae, Pseudomonas,

Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteriai, Salmonella, Shigella

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