Streptococcus iniae

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Streptococcus iniae
Phase contrast micrograph of S. iniae colonies stained with streptavidin-FITC
Scientific classification Edit this classification
Domain: Bacteria
Phylum: Bacillota
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus
Species:
S. iniae
Binomial name
Streptococcus iniae
Pier, 1976[1]
Synonyms

Streptococcus shiloi (Eldar, 1994)

Streptococcus iniae is a

septicemia
.

S. iniae has occasionally produced infection in humans, especially fish handlers of Asian descent. Human infections include sepsis, toxic shock syndrome, and inflammation of the skin, intervertebral discs, or inner layer of the heart. Identifying S. iniae in the laboratory can be difficult, since the conventional methods used to identify streptococci yield insufficient results. It cannot be grouped by the Lancefield antigen method typically used to categorize Streptococcus species. The two known serotypes can be distinguished biochemically by differences in enzyme activity. Several antibiotics have been used to treat S. iniae infections.

History

Streptococcus iniae was first isolated in 1972, from subcutaneous abscesses in a captive specimen of Amazon river dolphin (Inia geoffrensis) suffering from an infection known as "golf ball disease". The bacterium was found to be sensitive to beta-lactam antibiotics, and the dolphin was treated successfully with penicillin and tylosin. The causative organism was recognized to be a new species of Streptococcus, and was given the name Streptococcus iniae in 1976.[1] Around this time, other streptococcal outbreaks occurred in Asia,[2][3] and the US;[4] some strains associated with the Japanese outbreaks[2][3][5] were later suggested to be S. iniae.[6]

In the 1980s, a purported new species of Streptococcus, named S. shiloi, was identified as one of the causes of an epidemic of

ATCC type S. iniae and recalculation of the G+C% content, was reclassified by the same group as a junior synonym of S. iniae.[8]

pulsed field gel electrophoresis patterns have been identified among human isolates.[9][15]

Identification

Hemolyses of Streptococcus spp.
(left) α-hemolysis; (middle) β-hemolysis; (right) γ-hemolysis (= nonhemolytic)

S. iniae may be easily misidentified (or not identified at all) by conventional automated microbiology systems.

restriction endonucleases and Southern blotted using species-specific oligonucleotide probes.[19] This method is more sensitive than 16S rDNA sequencing, as in addition to species differentiation, it can be used to differentiate between strains. Ribotyping was used in 1997 to differentiate between Israeli and American strains, thus ruling out the possibility of an epidemiological link between outbreaks in the two countries.[17]

S. iniae is

sodium hippurate, and does not grow in bile esculin agar.[21] It does not express any of the known Lancefield antigens.[21]

Serotypes

Two

arginine dihydrolase activity has been used to distinguish between serotypes (serotype I is positive),[23] though proposed hyperencapsulation of serotype II may represent the most significant functional difference between the two types.[24]

Role in disease

In fish

Epizootics of S. iniae infection in rainbow trout (healthy specimen pictured) have occurred in Israel and Japan.[2][25]

S. iniae is highly pathogenic in

Common carp (Cyprinus carpio), channel catfish (Ictalurus punctatus), and goldfish (Carassius auratus) appear to be resistant.[27] Fish raised in intensive aquaculture operations and subject to environmental stressors (i.e. suboptimal temperature, poor water quality, crowding, handling, etc.) are most prone to S. iniae infection.[30][31][32][33][34] Wild fish populations located both near[10][35] and far from[36]
aquaculture operations have also proven susceptible to S. iniae infection.

The site of S. iniae infection and its clinical presentation vary from species to species. In tilapia, S. iniae causes meningoencephalitis, with symptoms including

septicemia and central nervous system damage. Symptoms are consistent with septicemia, and include lethargy and loss of orientation (as in tilapia), exophthalmia, corneal opacity, and external and internal bleeding.[25][37]

In humans

See also: Cutaneous Streptococcus iniae infection
Most human S. iniae infections are associated with injuries while handling live or fresh infected fish.

S. iniae can cause

bacteremic cellulitis, but has been known to cause endocarditis, meningitis, osteomyelitis, and septic arthritis.[15] The first recognized cases of human infection occurred in Texas in 1991 and in Ottawa in 1994, but the sources of infection were not determined.[38] Human infection with S. iniae was also identified in Toronto between 15 and 20 December 1995, when three Asian patients were admitted to a hospital with cellulitis of the hand after injuring themselves while handling raw fish. All three were found to have bacteremia, initially attributed to Streptococcus uberis, but later correctly identified as S. iniae.[14][26] In February 1996, a Chinese man was admitted to the same hospital with sepsis one week after preparing a fresh whole tilapia, and was also diagnosed with S. iniae bacteremia.[14][26] A subsequent epidemiological investigation found other cases in the Toronto area, for a total of nine patients; all were of Asian descent and all had handled raw fish (mostly tilapia) before developing infection.[14] Other cases were later identified in the United States and elsewhere in Canada, and have since been reported in Asia (Hong Kong, Taiwan, and Singapore).[15][39][40] Asian descent is a common trend in the majority of invasive human cases, but it is unknown whether this is due to inherent differences in immunity or because of cultural differences in the fish preparation which lend themselves to a higher incidence of infection.[27]

Control and treatment

Several measures can be taken to control infection in aquaculture once an S. iniae outbreak has been confirmed. Decreasing the quantity of feed given to fish has been shown to reduce mortality rates, as the uptake of bacteria in water is expedited by feeding. Decreasing the density of the fish stock increases survival by reducing injury to fish and lowering the general stress level in the population. Lowering the water temperature and keeping optimal oxygen levels has also been shown to reduce stress to fish and inhibit bacterial growth.[33]

A 2005 study showed the potential for using

Aeromonas sobria as a potential candidate for control of S. iniae and L. garvieae infections in aquaculture. A. sobria, given live in the feed, protected the trout when challenged with S. iniae or L. garvieae.[41]

Several antibiotics have been used successfully to treat S. iniae infection in fish. Enrofloxacin, a quinolone antibiotic, has been used to great effect in hybrid striped bass (Morone chrysops × M. saxatilis), although evidence suggested the development of a resistant strain during this trial.[42] Amoxicillin, erythromycin, furazolidone, and oxytetracycline have also been used (the last with varying success, only in barramundi).[27] Vaccination against S. iniae has been attempted with limited success as it only provides up to 6 months' immunity.[27][43]

co-trimoxazole (MICs 0.25 μg/ml); all nine patients were treated with parenteral beta-lactam antibiotics and recovered uneventfully.[14][26] A study of isolates submitted to the Centers for Disease Control and Prevention between 2000 and 2004 found all to be sensitive to beta-lactams, macrolides, quinolones, and vancomycin.[9]

References

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External links

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