Pathogenicity and role in the development of infections
Depending on their degree of danger to the organism, cocci are divided into two main groups:
- Pathogenic. Cause specific infectious diseases. These include, for example, gonococci and meningococci.
- Opportunistic. The vast majority of cocci belong to this category.
Normally, many opportunistic species coexist peacefully with humans as part of the normal microflora (normoflora) inhabiting various body biotopes. However, when the body is weakened or immunity is compromised (primary or secondary immunodeficiency), they can cause disease.
Most commonly, infection develops via the mechanism of translocation: the microorganism moves from its usual habitat to unusual tissues, where it provokes a purulent-inflammatory process. Due to this ability to cause suppuration, cocci are often called pyogenic (pus-forming) bacteria.
Diversity of aerobic Gram-positive cocci
This group includes numerous genera whose members are widely distributed in nature, constitute part of the normoflora, and can act as opportunistic pathogens.
Key genera:
- Staphylococcus;
- Streptococcus;
- Aerococcus;
- Leuconostoc;
- Pediococcus;
- Lactococcus;
- Gemella.
In addition to these, the coccus group includes enterococci, pneumococci, peptococci, Neisseria (Gram-negative), and Veillonella.
Characteristics of saprophytic and opportunistic genera
Some genera of cocci are more frequently found in the environment (air, water, plants) and on food products, but under certain conditions acquire clinical significance.
| Genus | Smear morphology | Physiology | Features |
|---|---|---|---|
| Aerococcus | Short chains or tetrads (groups of 4 cells), large (1–2 µm) | Microaerophiles | May cause hospital-acquired (nosocomial) infections. Resistant to heat and bile. |
| Leuconostoc | Intertwining chains with a thick capsule | Facultative anaerobes | Resistant to the antibiotic vancomycin and bile. Contaminate food products. |
| Pediococcus | Tetrads (cubic clusters) | Microaerophiles | Widely distributed on food products. |
| Lactococcus | In pairs or short chains | Facultative anaerobes | Inhabit plants and food products. |
| Gemella | Single or in pairs | Facultative anaerobes | Inhabit the mucous membranes of the mouth and GI tract. Produce $\beta$-hemolysis on blood agar. |
All these bacteria are non-motile and incapable of forming spores (asporogenous).
Briefly: answers to topical questions
Which bacteria belong to pathogenic cocci?
Pathogenic cocci include bacteria that cause specific diseases. According to sources, this group includes:
- Meningococci (genus Neisseria) — Gram-negative diplococci that are responsible for over 50% of primary purulent meningitis cases in children over 2 years of age.
- Gonococci — along with meningococci, are cited as a classic example of pathogenic cocci causing specific infections.
Which cocci are Gram-negative?
According to sources, Gram-negative cocci include:
- Neisseria (genus Neisseria): meningococcus is described as a Gram-negative diplococcus belonging to the genus Neisseria.
- Veillonella (genus Veillonella) — anaerobic Gram-negative coccoid bacteria; clinically significant species: V. atypica, V. dispar, V. parvula. They are arranged in pairs, short chains, or singly, and morphologically resemble Neisseria.
Which genera of cocci are obligate anaerobes?
The cited sources do not explicitly list genera of cocci designated specifically as obligate anaerobes.
At the same time, the sources highlight anaerobic cocci:
- Veillonella (genus Veillonella) — anaerobic Gram-negative coccoid bacteria.
- Peptococci (genus Peptococcus) — clinically significant anaerobic Gram-positive cocci.
- Peptostreptococci (genus Peptostreptococcus) — clinically significant anaerobic Gram-positive cocci.
- Finegoldia (genus Finegoldia) — clinically significant anaerobic Gram-positive cocci.
What purulent-inflammatory diseases are caused by pyogenic cocci?
Pyogenic (pus-forming) cocci are the main causative agents of purulent-inflammatory processes. According to sources, cocci can be associated with the following pathologies:
- Primary purulent meningitis — meningococcus causes over 50% of primary purulent meningitis cases in children over 2 years of age.
- Brain and lung abscesses, meningitis, aspiration pneumonia — indicated as typical pathologies for anaerobic Gram-positive cocci.
- Purulent-inflammatory processes of the maxillofacial region — indicated for anaerobic Gram-positive cocci.
- Purulent pulpitis — hemolytic streptococcus and Staphylococcus aureus are indicated for the purulent form.
- Purulent osteomyelitis — pathogens include staphylococci, streptococci, and anaerobic cocci.
- Wound infections, postpartum endometritis, chronic suppurative lung diseases — indicated for Peptostreptococcus anaerobius.
To remember that opportunistic cocci cause purulent inflammation, recall the term «pyogenic» (from Greek pyon — pus, genesis — to produce).
Class and exam questions
Are all cocci Gram-positive?
No. The cocci group is tinctorially heterogeneous: it includes both Gram-positive (e.g., staphylococci) and Gram-negative (e.g., Neisseria) bacteria.
How do cocci respire?
The types of respiration in cocci vary to the maximum extent. Obligate aerobes, microaerophiles, facultative and even obligate anaerobes are found.
Under what condition do opportunistic cocci cause disease?
The disease occurs when the host organism's resistance decreases (immunodeficiencies, debilitated state) and the microbe relocates (translocates) to a biotope atypical for it.
What else to study on this topic
- Morphological and cultural features of the genus Staphylococcus
- Classification and pathogenicity factors of the genus Streptococcus
- Gram-negative cocci: characteristics of the genus Neisseria
- Obligate anaerobic cocci: peptococci and peptostreptococci
- Mechanisms of resistance development in pyogenic cocci
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