Streptococci

Position in the phylogenetic tree

Streptococci belong to the domain Bacteria. Unlike members of the domain Eukarya (protozoa, fungi, plants, and animals), they are prokaryotes, meaning they lack a membrane-bound cell nucleus with a nucleolus. They are also fundamentally distinguished from the domain Archaea by the presence of peptidoglycan in their cell envelope and the ability to cause infectious diseases in humans (no known pathogens are found among archaea).

Within the domain Bacteria, classification is based on cell wall architecture and genetic data. The genus Streptococcus is part of the major phylum Firmicutes, which predominantly comprises Gram-positive microorganisms. Further down, they belong to the class Bacilli. It is important to note that this class includes both spore-forming bacteria (such as the genus Bacillus) and non-spore-forming forms, which include spherical cocci—staphylococci, pneumococci, and streptococci.

Morphology and cell wall structure

Morphologically, streptococci are cocci—spherical bacteria. Based on their cell envelope structure, they are classified as Gram-positive bacteria (Gram+).

Their cell wall is very thick and has a relatively simple structure. It consists almost entirely of a robust layer of peptidoglycan. A unique component of this layer is N-acetylmuramic acid. Unlike Gram-negative bacteria (such as meningococci, gonococci, or veillonellae), streptococci lack an additional outer membrane composed of phospholipids, lipoproteins, and lipopolysaccharides.

It is this envelope architecture that determines the behavior of bacteria during differential Gram staining. Due to the thick peptidoglycan layer, streptococci retain the stain effectively and acquire a characteristic blue-violet color (whereas Gram-negative bacteria stain red).

Cytological features

As typical prokaryotes, streptococci possess a number of specific intracellular characteristics:

• Genetic apparatus: the cytoplasm contains 70S-type ribosomes, which differ from the ribosomes of eukaryotic cells.
• Absence of membrane-bound organelles: the cell lacks mitochondria, the Golgi apparatus, the endoplasmic reticulum, or lysosomes, which are characteristic of protozoa and fungi.
• Biochemical composition: the bacterial structure contains compounds that are completely absent in the human body. These include D-isomers of amino acids and diaminopimelic (meso-diaminopimelic) acid.
• Absence of chitin: unlike members of the kingdom Eumycota (true fungi), the cell wall of streptococci does not contain chitin.

Although some Gram-positive bacteria are capable of forming endospores to survive extreme conditions (desiccation, UV irradiation), streptococci lack this ability—they are exclusively vegetative cells.

Medical significance and therapeutic targets

Pathogenic streptococci possess diverse biological properties, secrete specific toxins, and have a complex antigenic structure (including various serogroups and serotypes, as seen in Streptococcus pneumoniae).

However, the unique structure of their cell wall makes them vulnerable to antibacterial therapy. Since peptidoglycan, N-acetylmuramic acid, D-amino acids, and diaminopimelic acid are completely absent in the human body, the enzymes responsible for bacterial cell wall biosynthesis serve as ideal targets for antibiotics. Drugs that block these enzymes exhibit high selective toxicity: they destroy streptococcal cells while causing no harm to host cells.

Laboratory diagnostics

To establish a definitive diagnosis in streptococcal infections, a combination of clinical microbiology methods is used. A crucial step is adhering to proper specimen collection protocols for bacteriological examination. Laboratories perform culture-based diagnostics to isolate the pathogen and study its cultural properties. For precise species identification of streptococci, methods assessing bacterial enzymatic activity are employed.

Class and exam questions

What else to study on this topic

• Classification and serogroups of streptococci
• Pathogenicity factors and bacterial toxins
• Antigens and serotypes of Streptococcus pneumoniae
• Culture diagnostics and specimen collection guidelines
• Methods for determining bacterial enzymatic activity

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.