Streptococcus minor

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Domain: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus
Species: S. Minor

Streptococcus minor is a Gram-positive, rod-shaped, very small bacterium found in the tonsils and gastrointestinal tracts of dogs, cats, and cows. It is found within the ''Streptococcus'' genus by analyzing the 16S rDNA sequence and most similarly resembles the ''Streptococcus ovis'' species.[1]. They range in size from .5mm to 1mm in diameter and arrange themselves into small groups. Colonies grow unpigmented, translucent, and alpha-hemolytic.[2]. S. Minor was discovered in 2004 in Belgium from samples obtained from the tonsils, anal swaps, and feces of dogs and from the tonsils of a cat and calf.


The genome of S. minor has about 1497 base pairs and linear DNA.[2]. Two representative strands were found by analyzing 16S rDNA gene sequence, LMG 21734 and LMG 21735.[3]. Streptococcal genome size is relatively small possible due to adaptation reasons such as reproductive efficiency and/or competitiveness for a new host environment.[4]. The genome of S. minor consists of around 40-41% GC content.[2]. The Streptococcus genus has a low GC content, other species in this genus have around 33%-43% GC content, with S. minor falling in the middle.[3]. The GenBank accession number for S. minor is AY232832 and the type of strain is ON59.[2].


Very little is known about this species, especially about its metabolic process. What is known is that S. minor growth is enhanced when placed in a CO2 environment. Almost all the species in the Streptococcus genus are facultatively anaerobic (catalase-negative) and some require addition CO2 for growth, like S. minor. S. minor is a chemoorganotrophic bacterium, which means it gets its energy from organic compounds, and has a fermentative metabolism.[3]. Carbohydrates are fermented to produce lactic acid. They grow on Edwards medium (Oxoid), but not on Slanetz and Bartley agar (Oxoid).[1]. This species grows best in 37°C, however growth was seen in both 25°C and 42°C temperatures.[1].

Characteristics of Streptococcus

Main characteristics of the genus Streptococcus include being either spherical or ovoid shape and less than 2 µm in diameter. The Streptococcus cells are arranged in chains or pairs, chain length varies based on species and strains and is also dependent on medium environment.[3]. They are gram-positive bacteria and contain peptidoglycan with a variety of carbohydrates, teichoic acid, and surface protein antigens. Several of S. minor closes relatives in this genus, S. oralis and S. suis, lack interpeptide bridge and have a directly cross-linked peptidoglycan.[3]. All of the species ferment glucose, however the pattern in which the ferment can be used to identify which species is which. Growth is increased a lot by the addition of glucose in the medium, but the decrease in pH levels will inhibit the growth of all species. Morphologically, Streptococcus species are associated with warm blooded animals and birds.[3].

Identifying Streptococcus Species

Biochemically testing is the main source of identification for these bacteria. S. minor was identified by a variety of tests: tRNA intergenic length polymorphism analysis (tDNA-PCR) and SDS-PAGE analysis of whole-cell proteins.[1]. tDNA-PCR was performed by using a primers T5A (59-AGTCCGGTGCTCTAACCA- ACTGAG-39) and fluorescently labelled T3B (59-AGGT- CGCGGGTTCGAATCC-39).[1]. tDNA spacer fragments were identified and utilized to differentiate between the other species. Further investigating was done by utilizing the PAGE analysis of whole cell proteins. Whole-cell protein profiles were compared with patterns that represented all currently described lactic acid bacteria in this genus.[1].

Biochemical Composition

API testing allows for the following characteristic of the S. minor. All strains tested positive for leucine arylamidase, hydrolysis of L-valine-AMC, L-phenylalanine- AMC, 4MU-a-D-glucoside, L-tryptophan- AMC, L-arginine-AMC, L-isoleucine-AMC, and aesculin.[1].

However, has tested negative in for activity of hippurate, pyrrolidonyl arylamidase, b-glucuronidase, b-galactosidase and alkaline phosphatase.[1]. This species has also tested negative for acid production from glycerol, erythritol, DL-arabinose, ribose, DL-xylose, adonitol, methyl b-xyloside, L-sorbose, rhamnose, dulcitol, inositol, methyl a-D-mannoside, methyl a-glucoside, melibiose, melezitose, xylitol, D-turanose, D-lyxose, DL-fucose, DL-arabitol, gluconate and, 2- and 5-ketogluconate.[1].

S. minor is strain-dependent for a-galactosidase and alkaline phosphatase activity, hydrolysis of p-nitrophenyl phosphate and acid production from sorbitol, amygdalin, arbutin, inulin, D-raffinose, starch and D-tagatose. Strains don’t react with Lancefield groups A, B, C, D, G, or F antisera.[1].

Recent Developments

The first case of S. minor was found in a 51-year-old woman who was bitten by a dog. Before this case, no other case of S. minor has ever been presented in a human. Sequencing 16S rDNA allowed for 100% confirmation that the identity of the bacteria was S. minor. The antimicrobial test showed that this bacterium was susceptible to penicillin, clindamycin, erythromycin and trimethoprim. The patient left the hospital with 10 days’ worth of amoxicillin and recovered well under this antibiotic. More information about the bacterium’s pathogenies are still unknown, however, this case suggests that it has potential to cause serious infections in humans if left untreated.[5].


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Vancanneyt, M., Devriese, L.A., De Graef, E.M., Baele, M., Lefebvre, K., Snauwaert, C., Vandamme, P., Swings, J., and Haesebrouck, F. (2004). Streptococcus minor sp. nov., from faecal samples and tonsils of domestic animals. Int. J. Syst. Evol. Microbiol. 54, 449–452.
  2. 2.0 2.1 2.2 2.3 Streptococcus minor strain LMG 21734 16S ribosomal RNA gene, partial s - Nucleotide - NCBI.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 "Streptococcus".Bergey's Manual of Systematics of Archaea and Bacteria. American Cancer Society. 2015. pp. 1–86. ISBN 978-1-118-96060-8.
  4. Gao, X.Y., Zhi, X.Y., Li, H.W., Klenk, H.P., and Li, W.J. (2014) Comparative genomics of the bacterial genus streptococcus illuminates evolutionary implications of species groups. PLoS One 9.
  5. Tré-Hardy, M., Saussez, T., Yombi, J.C., and Rodriguez-Villalobos, H. (2016). First case of a dog bite wound infection caused by Streptococcus minor in human. New Microbes New Infect. 14, 49–50.