Cocci Bacteria Cocci Bacteria – Definition, Types, and Common Diseases
Bacteria come in various shapes and sizes, each adapted to different environments and functions. Among these, cocci bacteria stand out for their characteristic spherical, oval, or round form. This guide explores their morphology, classification, role in causing diseases, and unique traits that make them an essential topic of study in microbiology.
Morphology of Bacteria
Before delving into cocci specifically, let us first understand the broad classification of bacterial shapes:
Cocci: Spherical or round-shaped bacteria (e.g., Streptococcus, Staphylococcus).
Bacilli: Rod-shaped bacteria (e.g., Escherichia coli).
Spirochaetes: Slender, spiral, or corkscrew-shaped bacteria (e.g., Treponema pallidum).
Vibrio: Comma-shaped bacteria (e.g., Vibrio cholerae).
Star-shaped, pleomorphic, and other less common forms.
Pleomorphic bacteria can change shape or size based on external conditions. One well-known example is Mycoplasma pneumoniae, which lacks a rigid cell wall, thus varying in form.
What are Cocci Bacteria?
Definition
The term “cocci” (singular: coccus) is derived from the Greek word kokkos, meaning “berry” or “seed”. Cocci bacteria are typically round or oval in shape, and they can appear alone, in pairs, chains, clusters, or other unique arrangements.
Common Arrangements of Cocci
Monococci – Single, isolated cocci.
Diplococci – Pairs of cocci (e.g., Neisseria gonorrhoeae).
Streptococci – Chains of cocci (e.g., Streptococcus mutans).
Staphylococci – Clusters of cocci resembling grape-like bunches (e.g., Staphylococcus aureus).
Tetrads – Groups of four cocci arranged in a square formation (e.g., Pediococcus).
Sarcina – Groups of eight cocci forming cubical pockets (e.g., Sarcina ventriculi).
These arrangements often result from how bacterial cells divide and whether daughter cells remain attached after division.
Gram-Positive vs Gram-Negative Cocci
Bacteria are commonly distinguished by Gram staining, a technique that reveals differences in cell wall structure:
Gram-Positive Cocci
Cell Wall Structure: Thick peptidoglycan layer that retains the crystal violet stain, appearing purple under the microscope.
Examples:
Staphylococci: Usually form clusters and are non-motile, non-sporing. A classic example is Staphylococcus aureus, about 1µm in diameter.
Streptococci: Typically form chains. While some are harmless members of our normal flora, others can be significant pathogens (e.g., Streptococcus pneumoniae).
Micrococci: Often form tetrads and are similar to staphylococci but differ in their metabolic pathways (oxidative vs fermentative).
Other Gram-positive cocci genera include Peptococcus, Parvimonas, Peptoniphilus, Blantia, Ruminococcus, Finegoldia, and Anaerococcus.
Gram-Negative Cocci
Cell Wall Structure: Thinner peptidoglycan layer surrounded by an outer membrane, which does not retain the crystal violet stain but takes up the counterstain (safranin), appearing pink/red.
Examples:
Anaeroglobus
Acidaminococcus
Veillonella
Megasphaera
Negativococcus
Gram-negative cocci can sometimes be overlooked but are clinically significant, especially in certain oral or intestinal infections.
Diseases Caused by Cocci Bacteria
Although many cocci are harmless or even beneficial members of our microbiome, certain strains can cause diseases in humans and animals.
Pneumococcus
Streptococcus pneumoniae is a Gram-positive diplococcus that colonises the respiratory tract.
Common infections include pneumonia, meningitis, and sepsis.
Vaccinations against pneumococcal infections are widely used to prevent severe illness.
Peptostreptococci
Small, Gram-positive anaerobic cocci often found in normal human flora.
Can lead to infections such as puerperal sepsis, pleuropulmonary disease, and brain abscesses (e.g., Peptostreptococcus anaerobius).
Staphylococcus aureus
A well-known Gram-positive coccus that forms grape-like clusters.
Commonly resides on the skin and in nasal passages.
Can cause skin infections (e.g., boils, impetigo, staphylococcal scalded skin syndrome), food poisoning, and even more severe conditions like toxic shock syndrome if it enters the bloodstream.
Some strains, like MRSA (Methicillin-Resistant Staphylococcus aureus), pose treatment challenges due to antibiotic resistance.
Additional Examples of Cocci-Related Conditions
Staphylococcal Food Poisoning: Occurs when toxins produced by certain Staphylococcus aureus strains contaminate food.
Streptococcal Throat Infections: Often caused by Streptococcus pyogenes, leading to pharyngitis or tonsillitis.
Meningococcal Disease: Although caused by a Gram-negative diplococcus (Neisseria meningitidis), it is a crucial example of how cocci can invade the bloodstream and central nervous system.
Similar-Sounding Terms to Avoid Confusion
Coccobacilli
Short, oval rod-shaped bacteria (e.g., Brucella).
Morphologically intermediate between bacilli and cocci.
Coccidioidomycosis
A fungal infection caused by Coccidioides immitis or Coccidioides posadasii.
Also known as “Valley Fever,” it primarily affects the lungs and is not related to cocci bacteria.
Coccidiosis
A parasitic disease caused by apicomplexan protozoans (e.g., Eimeria, Cryptosporidium).
Affects the intestinal tract of animals such as poultry, dogs, and cats, but not related to bacterial cocci.
Mycoplasmas
Cell wall-deficient bacteria that do not have a fixed shape (e.g., Mycoplasma pneumoniae).
Not structurally classified as cocci, bacilli, or spirilla.
In Summary
Cocci bacteria are a diverse group, ranging from helpful inhabitants of our body to formidable pathogens capable of causing severe diseases. Their spherical shape, varied arrangements (chains, clusters, pairs), and Gram-staining properties offer valuable clues about their behaviour, classification, and potential pathogenicity. By studying cocci and understanding the preventive measures against infections, we equip ourselves to maintain better health while appreciating the remarkable diversity of the microscopic world.
For more interactive resources, study tips, and expert guidance on Cocci Bacteria and other essential biology topics, explore Vedantu—your trusted partner in learning. Our mission is to simplify complex concepts, making them accessible to students of all grades, and ensuring a comprehensive, enjoyable educational experience.
FAQs on Cocci Bacteria
1. Are all cocci bacteria harmful?
No. Many cocci (such as some Streptococcus and Micrococcus species) are harmless and even beneficial. Pathogenicity depends on the specific strain and its ability to cause disease under certain conditions.
2. Why do cocci sometimes form clusters and sometimes chains?
Cocci form particular arrangements based on how they divide and whether daughter cells remain attached after division. For instance, if the division occurs in multiple planes, clusters (like staphylococci) can form; if it occurs in a single plane, you may see chains (like streptococci).
3. How can I distinguish Gram-positive cocci from Gram-negative cocci?
A Gram stain test can be performed in a lab. Gram-positive cocci retain the crystal violet stain (appear purple), while Gram-negative cocci lose the primary stain but take up the counterstain (appear pink/red).
4. What makes Staphylococcus aureus particularly concerning?
While it is a common inhabitant of the human skin and nasal passages, certain strains can produce toxins or become resistant to multiple antibiotics (like MRSA), making infections challenging to treat.
5. Which preventive measures help reduce cocci-related infections?
Good hygiene practices (frequent hand washing), responsible antibiotic usage, and following vaccination schedules (e.g., pneumococcal vaccines) are effective ways to lower the risk of infections by pathogenic cocci.
