Coryneform bacteria are a diverse collection of rod-shaped, Gram-positive, and non-spore-forming microorganisms. The name “coryneform” comes from the Greek word koryne (club), describing the distinctive, club-shaped morphology many species exhibit. This broad group is ubiquitous, found nearly everywhere from deep soil to the human body. The significance of these bacteria is wide-ranging, encompassing species that are harmless or beneficial, alongside others capable of causing serious disease.
Defining Characteristics and Environment
Coryneform bacteria are categorized within the phylum Actinomycetota, sharing a high DNA G+C content with bacteria like Mycobacterium and Nocardia. Under a microscope, these bacilli frequently display an irregular, pleomorphic shape, often appearing slightly curved or club-shaped. A striking feature is their arrangement after division, frequently forming angular patterns resembling the letters V, L, or a “Chinese letter” arrangement, or grouping in palisades.
These bacteria are generally aerobic, but many species are facultative anaerobes, able to switch to fermentation when oxygen is scarce. They thrive in vast natural habitats, including soil, water, and various plant and animal hosts, utilizing diverse carbon sources.
Commensal Roles and Industrial Importance
Many coryneform species are harmless commensals, forming an intrinsic part of the human skin microbiome, especially in moist areas like the armpits and groin. Here, they help maintain the skin’s ecosystem by competing with harmful organisms and contributing to the slightly acidic “acid mantle,” which discourages pathogen colonization.
Beyond biological roles, certain coryneform species, notably Corynebacterium glutamicum, hold immense value in biotechnology. This non-pathogenic species is a powerhouse in industrial fermentation due to its ability to overproduce specific metabolites. It is used globally for the commercial production of amino acids, particularly L-glutamic acid and L-lysine, which are used in food, feed, and pharmaceutical products.
Major Pathogens and Associated Illnesses
Despite the many harmless species, the genus Corynebacterium includes highly pathogenic members, with Corynebacterium diphtheriae being the most infamous. This bacterium causes diphtheria, a contagious disease primarily affecting the respiratory tract. Its ability to cause disease is linked to the diphtheria toxin, an exotoxin produced only if the bacterium is infected by a bacteriophage carrying the toxin gene.
The diphtheria toxin is an A-B toxin that disrupts protein synthesis in host cells, leading to cell death. In the throat, this results in a thick, grayish pseudomembrane covering the tonsils and airways, potentially causing breathing difficulties. The toxin can also spread through the bloodstream, leading to systemic complications such as heart failure and nerve damage, which may cause paralysis. Less common is cutaneous diphtheria, where the bacterium infects skin lesions, resulting in chronic, non-healing ulcers.
Other species, often called non-diphtherial corynebacteria or “diphtheroids,” function as opportunistic pathogens. These organisms are typically part of the normal human flora but cause serious infections when the host’s immune system is weakened or foreign medical devices are present. Examples include C. jeikeium and C. striatum, which are associated with infections like endocarditis and bacteremia, particularly in patients with vascular catheters or hematological disorders. C. urealyticum is specifically known for causing genitourinary tract issues, such as encrusted cystitis, due to its ability to hydrolyze urea.
Diagnosis and Management of Infection
Diagnosis of coryneform infection typically begins with laboratory culture and Gram staining of a clinical specimen. The distinct, club-shaped, Gram-positive rods provide an initial indication, but species-level identification is required to determine clinical significance, often using biochemical assays or mass spectrometry. For suspected diphtheria, confirming the presence of C. diphtheriae is not sufficient; the isolate must also be tested for its ability to produce the diphtheria toxin.
The gold-standard method for detecting toxin production is the Elek test, an immunoprecipitation assay that visually confirms the interaction between the secreted toxin and a specific antitoxin. While molecular tests like PCR can quickly detect the tox gene, a positive result must be confirmed by the Elek test, as some strains carry the gene but do not produce the active toxin. Management of non-diphtherial infections relies on identifying the strain and determining antibiotic susceptibility, as many opportunistic coryneform species have developed resistance to common antibiotics, often requiring treatment with drugs like vancomycin.
For confirmed diphtheria, treatment is a two-pronged approach. The patient must receive diphtheria antitoxin (DAT) as soon as possible to neutralize any circulating toxin that has not yet bound to host tissues. This is coupled with antibiotics, such as penicillin or macrolides, to halt the growth of the bacteria and stop further toxin production. The widespread use of the diphtheria toxoid vaccine, which generates immunity against the toxin itself, has made the disease rare in many parts of the world and remains the most effective form of prevention.