biology

Streptococcus

Streptococcus is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Streptococcus rather than just read about it. In short: Streptococcus (from Ancient Greek στρεπτός (streptós) 'twisted' and κόκκος (kókkos) 'grain') is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a single axis, thus when growing they tend to form pairs or chains, which may appear bent or twisted.

Key takeaways

Streptococcus belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Streptococcus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Streptococcus from memory before moving on to harder problems.

Reference excerpt

Streptococcus (from Ancient Greek στρεπτός (streptós) 'twisted' and κόκκος (kókkos) 'grain') is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a single axis, thus when growing they tend to form pairs or chains, which may appear bent or twisted. This differs from staphylococci, which divide along multiple axes, thereby generating irregular, grape-like clusters of cells. Most streptococci are oxidase-negative and catalase-negative, and many are facultative anaerobes (capable of growth both aerobically and anaerobically). The term was coined in 1877 by Viennese surgeon Albert Theodor Billroth (1829–1894), from Ancient Greek στρεπτός (streptós), meaning "twisted", and κόκκος (kókkos), meaning "grain". In 1984, many bacteria formerly grouped in the genus Streptococcus were separated out into the genera Enterococcus and Lactococcus. Currently, over 50 species are recognised in this genus. Streptococcus species play a large role in human health and disease. Most exist as commensally as parts of the human microbiome, for example in the mouth, skin, intestine, and upper respiratory tract. Some exist as pathogens and cause infections collectively known as streptococcosis, such as streptococcal pharyngitis (strep throat), pink eye, meningitis, bacterial pneumonia, endocarditis, erysipelas, and necrotizing fasciitis (the 'flesh-eating' bacterial infections). A few are opportunistic, being able to switch between friend and foe. Streptococcus are also used in the production of food, with S. thermophilus being used to make Emmentaler ("Swiss") cheese and yogurt.

Molecular taxonomy and phylogenetics

Streptococci have been divided into six groups on the basis of their 16S rDNA sequences: S. anginosus, S. gallolyticus, S. mitis, S. mutans, S. pyogenes and S. salivarius. The 16S groups have been confirmed by whole genome sequencing (see figure). The important pathogens S. pneumoniae and S. pyogenes belong to the S. mitis and S. pyogenes groups, respectively, while the causative agent of dental caries, Streptococcus mutans, is basal to the Streptococcus group.

Recent technological advances have resulted in an increase of available genome sequences for Streptococcus species, allowing for more robust and reliable phylogenetic and comparative genomic analyses to be conducted. In 2018, the evolutionary relationships within Streptococcus was re-examined by Patel and Gupta through the analysis of comprehensive phylogenetic trees constructed based on four different datasets of proteins and the identification of 134 highly specific molecular signatures (in the form of conserved signature indels) that are exclusively shared by the entire genus or its distinct subclades. The results revealed the presence of two main clades at the highest level within Streptococcus, termed the "Mitis-Suis" and "Pyogenes-Equinus-Mutans" clades. The "Mitis-Suis" main clade comprises the Suis subclade and the Mitis clade, which encompasses the Angiosus, Pneumoniae, Gordonii and Parasanguinis subclades. The second main clade, the "Pyogenes-Equinus-Mutans", includes the Pyogenes, Mutans, Salivarius, Equinus, Sobrinus, Halotolerans, Porci, Entericus and Orisratti subclades. In total, 14 distinct subclades have been identified within the genus Streptococcus, each supported by reliable branching patterns in phylogenetic trees and by the presence of multiple conserved signature indels in different proteins that are distinctive characteristics of the members of these 14 clades. A summary diagram showing the overall relationships among the Streptococcus based on these studies is depicted in a figure on this page.

Medical taxonomy

In clinical microbiology, species of streptococci are classified by phenotype using fast and simple biochemical tests. The first step is to test for hemolytic properties. Alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells, giving it a greenish color on blood agar. Beta-hemolytic species cause complete rupture of red blood cells. On blood agar, this appears as wide areas clear of blood cells surrounding bacterial colonies. Gamma-hemolytic species cause no hemolysis. Beta-hemolytic streptococci are further classified by Lancefield grouping, a serotype classification (that is, describing specific carbohydrates present on the bacterial cell wall). The 21 described serotypes are named Lancefield groups A to W (excluding E, I and J). This system of classification was developed by Rebecca Lancefield, a scientist at Rockefeller University. In the medical setting, the most important groups are the alpha-hemolytic streptococci S. pneumoniae and Streptococcus viridans groups, and the beta-hemolytic streptococci of Lancefield groups A and B (also known as "group A strep" and "group B strep"). Table: Medically relevant streptococci

Alpha-hemolytic When alpha-hemolysis (α-hemolysis) is present, a blood based agar under the colony will appear dark and greenish due to the conversion of hemoglobin to green biliverdin. Streptococcus pneumoniae and a group of oral streptococci (Streptococcus viridans or viridans streptococci) display alpha-hemolysis. Alpha-hemolysis is also termed incomplete hemolysis or partial hemolysis because the cell membranes of the red blood cells are left intact. This is also sometimes called green hemolysis because of the color change in the agar.

Pneumococci S. pneumoniae (sometimes called pneumococcus), is a leading cause of bacterial pneumonia and the occasional etiology of otitis media, sinusitis, meningitis, and peritonitis. Inflammation is thought to be the major cause of how pneumococci cause disease, hence the tendency of diagnoses associated with them to involve inflammation. They possess no Lancefield antigens.

The viridans group: alpha-hemolytic The viridans streptococci are a large group of commensal bacteria that are either alpha-hemolytic, producing a green coloration on blood agar plates (hence the name "viridans", from Latin vĭrĭdis, green), or nonhemolytic. They possess no Lancefield antigens. The "viridans" group is a wastebasket taxon. It includes representatives of all six traditional 16S groups of the genus.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Streptococcus

Start with the simplest possible case. Write down what Streptococcus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Streptococcus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Streptococcus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Streptococcus

In research: Streptococcus appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Streptococcus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Streptococcus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bacteria genera, Gram-positive bacteria, Gut flora bacteria, so understanding it makes those chapters shorter.
In everyday life: Look for Streptococcus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Streptococcus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Streptococcus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Streptococcus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Streptococcus in simple terms?

Why does Streptococcus matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Streptococcus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Streptococcus.