Atypical bacteria are

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

that do not get colored by gram-staining but rather remain colorless: they are neither Gram-positive nor Gram-negative. These include the

Chlamydiaceae The Chlamydiaceae are a family of gram-negative bacteria that belongs to the phylum Chlamydiota, order Chlamydiales. Chlamydiaceae species express the family-specific lipopolysaccharide epitope αKdo-(2→8)-αKdo-(2→4)-αKdo (previously ca ...

Legionella ''Legionella'' is a genus of pathogenic gram-negative bacteria that includes the species '' L. pneumophila'', causing legionellosis (all illnesses caused by ''Legionella'') including a pneumonia-type illness called Legionnaires' disease and a mil ...

and the

Mycoplasmataceae Mycoplasmataceae is a family of bacteria in the order Mycoplasmatales. This family consists of the genera '' Mycoplasma'' and '' Ureaplasma''. In 1967, the order Mycoplasmatales was incorporated into the class Mollicutes. Many species are sexua ...

(including mycoplasma and

ureaplasma ''Ureaplasma'' is a genus of bacteria belonging to the family Mycoplasmataceae. As the name imples, ''Ureaplasma'' is urease positive. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenc ...

); the Spirochetes and

Rickettsiaceae The Rickettsiaceae are a family of bacteria. The genus '' Rickettsia'' is the most prominent genus within the family. The bacteria that eventually formed the mitochondrion (an organelle in eukaryotic cells) is believed to have originated from ...

are also often considered atypical. Gram-positive bacteria have a thick

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...

layer in their cell wall, which retains the

crystal violet Crystal violet or gentian violet, also known as methyl violet 10B or hexamethyl pararosaniline chloride, is a triarylmethane dye used as a histological stain and in Gram's method of classifying bacteria. Crystal violet has antibacterial, antif ...

during Gram staining, resulting in a purple color. Gram-negative bacteria have a thin peptidoglycan layer which does not retain the crystal violet, so when

safranin Safranin (Safranin O or basic red 2) is a biological stain used in histology and cytology. Safranin is used as a counterstain in some staining protocols, colouring cell nuclei red. This is the classic counterstain in both Gram stains and endos ...

is added during the process, they stain red. The Mycoplasmataceae lack a peptidoglycan layer so do not retain crystal violet or safranin, resulting in no color. The Chlamydiaceae contain an extremely thin peptidoglycan layer, preventing visible staining. Ricketsiaceae are technically Gram-negative, but are too small to stain well, so are often considered atypical. Peptidoglycans are the site of action of

beta-lactam antibiotics β-lactam antibiotics (beta-lactam antibiotics) are antibiotics that contain a beta-lactam ring in their chemical structure. This includes penicillin derivatives (penams), cephalosporins and cephamycins (cephems), monobactams, carbapenems and ...

such as

penicillins Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from ''Penicillium'' moulds, principally '' P. chrysogenum'' and '' P. rubens''. Most penicillins in clinical use are synthesised by P. chrysogenum using ...

and

cephalosporins The cephalosporins (sg. ) are a class of β-lactam antibiotics originally derived from the fungus ''Acremonium'', which was previously known as ''Cephalosporium''. Together with cephamycins, they constitute a subgroup of β-lactam antibiotics ...

, so mycoplasma are naturally resistant to these drugs, which in this sense also makes them “atypical” in the treatment of their infections.

Macrolides The Macrolides are a class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. Macr ...

such as erythromycin however, are usually effective in treating atypical bacterial infections. Finally, some of these bacteria can cause a specific type of

pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severi ...

referred to as

atypical pneumonia Atypical pneumonia, also known as walking pneumonia, is any type of pneumonia not caused by one of the pathogens most commonly associated with the disease. Its clinical presentation contrasts to that of "typical" pneumonia. A variety of microorgan ...

. That is not to say that atypical pneumonia is strictly caused by atypical bacteria, for this disease can also have a fungal, protozoan or viral cause. Through a recent study on analyzing synergistic interactions between the influenza viruses and atypical bacteria, it was stated that there have been findings of interaction between the two most prominent strains ''C. Pneumoniae'' and ''M. Pneumoniae'' with the influenza virus. This was labeled and discussed as a coinfection in correlation to the influenza virus.

References

Bacteriology Bacteria organized by reaction to stain {{microbiology-stub

See also

References