A diphtheroid is a bacterium that resembles the species causing diphtheria but does not produce the diphtheria toxin.
Explanation
Members of the genus Corynebacterium are Gram-positive, pleomorphic rods that live on the skin and mucous membranes of humans and animals. These bacteria are often called diphtheroids as they resemble Corynebacterium dior the potent toxinhtheriae yet lack the gene for the potent toxin. Diphtheroids are part of the normal flora of the throat, nose and skin and rarely cause disease in healthy people. Some species such as C. striatum and C. jeikeium can cause opportunistic infections in patients with weakened immunity. In culture, diphtheroids appear as club‑shaped rods that may form palisades or Chinese letter arrangements. They are catalase‑positive, non‑motile and often produce an earthy smell. Molecular studies show that the term “diphtheroids” covers a diverse group of corynebacteria unrelated to the toxigenic species. They resemble the pathogen yet are generally harmless, so they can be ignored in throat cultures unless the patient has symptoms consistent with diphtheria. Laboratory tests such as biochemical reactions and the Elek test for toxin production ca distinguish diphtheroids from the toxin‑producing bacterium. Some corynebacteria
have industrial importance, with species used for the production of amino acids, vitamins and steroids. Their resistance to drying and disinfectants makes them hardy contaminants in hospital environments.
Uses and significance
Most diphtheroids live harmlessly on the body, but they can cause infections in specific settings. C. jeikeium is known for causing catheter‑related bloodstream infections in neutropenic patients. C. urealyticum can colonise the urinary tract and cause alkaline encrusted cystitis. Accurate identification leads to appropriate treatment, as corynebacteria vary in antibiotic susceptibility. Diphtds are also studied for their ability to transform steroids and produce enzymes useful in biotechnology.
An understanding of diphtheroids helps in interpreting microbiological cultures and distinguishing pathogenic from harmless bacteria. Their presence as normal flora shows the diversity of microbes that coexist with us and occasionally become opportunistic pathogens.
Related Terms: Corynebacterium, Gram-positive bacteria, Commensal bacteria, Diphtheria, Opportunistic infection