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Corynebacterium striatum is a prokaryotic bacterium that is a member of the Corynebacterium genus although is classified as non-diphtheritic. They are a gram-positive bacterium that assume a coccobacillus structure, otherwise known as a ‘club-like’ morphology. It is non-lipophilic and undergoes aerobic respiration and is also a facultative anaerobe it is catalase negative and oxidase positive glucose and sucrose fermenter.

It is generally found as a ubiquitous microorganism, and, as a commensal of humans, colonising the nasopharynx. It has recently been recognised as an emerging pathogen although the genus of Corynebacterium is not usually considered to be pathogenic. Particularly in the context of human disease Corynebacterium striatum is generally considered an opportunistic pathogenic, particularly in a nosocomial setting. It has been recorded to infect the skin, upper and lower respiratory tract and even disseminate resulting in septicaemia. Recent interest has been sparked in the microorganism as it is known to be resistant to and gaining resistances to many antibiotics.

History
Corynebacterium striatum is recognised as its own distinct organism, as a member of the Corynebacterium genus. Scientific papers dating back until approximately 1980 recount cases of commensal Corynebacterium striatum contaminating samples from sites of infections. After this it was formally defined in 1995. After this time Corynebacterium striatum was found to be the cause of disease. Early clinical testing of hospital patients found that infection generally only occurred in immunocompromised individuals or those that had some for of prosthetic device permanently or intermittently fitted. Not long after, researchers began to propose the notion that Corynebacterium striatum was the cause of disease even in patient that did not meet these criteria.

It is otherwise called a diphtheroid or coryneform due to its close phylogenetic relationship with diphtheria causing bacterium Corynebacterium diphtheriae.

Physical
Corynebacterium striatum is a gram-positive bacterium, meaning they have a thin external peptidoglycan cell wall structure. They have been described as having an irregular shape, that is scientifically termed a coccobacillus. Upon observation this appears as a ‘club-like’ shape with a larger more cocci shaped end attached to a rod-shaped bacterium. C striatum colonies are able to be plated in vitro, growing on Blood Agar the colonies will appear as small (1-2mm diameter), white, moist, and smooth.

Molecular/Biochemical
Corynebacterium striatum can be differentiated from other Corynebacterium types based on its ability to ferment glucose and sucrose, but inability to ferment maltose. In comparison to other members of the Corynebacterium genus it ferments sugars rapidly. It is also able to hydrolyse tyrosine. It does not metabolize urease, can hydrolyze esculin, and can also ferment mannitol and xylose.

Virulence Factors
The organism itself possesses few virulence factors, giving it the title of an opportunistic colonizer as opposed to a true pathogen.

Antibiotic-Resistance
Antibiotic resistance is the acquisition of resistance to antibiotic treatments through either horizontal gene transfer or genetic mutation. The acquisition of such characteristics by Corynebacterium striatum is relevant to its occurrence as a pathogen.

One study found that 93.7% of Corynebacterium striatum strains isolated showed resistance to at least one of the antimicrobial compounds tested. In this experiment, 82.5% of strains expressed resistance to the antibiotic penicillin . Researchers deduced that due to the long-term exposure of Corynebacterium striatum to Penicillin resistance had been acquired by most isolates. In the same experiment, multi drug resistance was observed in 49.2% of strains.

Corynebacterium striatum has been found to carry the bla gene. This gene encodes a class A β-lactamase. β-lactamase are a group of antimicrobial enzymes that work to counter the effect of β-lactam antibiotics such as ampicillin and penicillin.

Further genes associated with antibiotic resistance include, but are not limited to: the gyrA gene which is attributed to resistance to the major antibiotic group fluoroquinolones, the aph(3′)-Ic gene imparting resistant to kanamycin, aph(3″)-Ib and aph(6)-Id causing resistant to streptomycin. Researchers also found that the erm(X) gene causes resistant to erythromycin.

Disease
Infection with Corynebacterium striatum was initially thought to occur through self-infection, transmitting the bacteria from a site where it persists as commensal and then allowing it to colonise in a situation as a pathogen. It is now understood that it can be transmitted person to person, particularly in a hospital setting.

While Corynebacterium infections are not common, when they have been observed it is in individuals with prosthetic devices or those who are immunosuppressed. When detected under these conditions it is considered a true pathogen.

Infection
Many cases of infection with Corynebacterium striatum have been documented with particular relevance to acquisition of infection in hospitals - otherwise known as a nosocomial infection. Corynebacterium striatum has been known to colonise prosthetics, particularly - heart valves, prosthetic joints and even intravenous apparatus such as catheters. Infections of this type have been described as a local infection or they can progress into a larger disseminated infection otherwise known as bacteraemia.

One particular outbreak at the Hospital Joan March, Mallorca, Spain, saw 21 individuals infected with Corynebacterium striatum . The individuals all suffered from chronic obstructive pulmonary disease (COPD) and as such were consistently being admitted to the hospital where they were treated by care staff with shared equipment. In this instance Corynebacterium striatum was causing infection in the respiratory tract of patients and detected in sputum samples.

Another study of infection with Corynebacterium striatum described a patient who was admitted to hospital for treatment of cardiac arres t in 2008. During her stay the patient was fitted with a central venous catheter through which she contracted bacteraemia of Corynebacterium striatum which resulted in her death . The patients age and immuno-compromised state resulting from pre-existing renal failure ultimately allowed for the establishment and dissemination of infection.

Treatment
Multidrug resistance is the main factor considered when treating disease caused by Corynebacterium striatum . Treatment with a mix of broad-spectrum antibiotics may thus be necessary. Further questions surrounding the implications of such treatment are thus raised.

More broadly prevention of initial infection, particularly in a hospital setting is a key element of treatment. Sterilising all surfaces, and prosthetics is an integral element of stopping disease establishment.