A coccus (plural cocci) is any bacterium or archaeon that has a spherical, ovoid, or generally round shape. Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped), bacillus (rod-shaped) and spiral (of which there are two types: spirillum and spirochete). Coccus refers to the shape of the bacteria, and can contain multiple genera, such as staphylococci or streptococci. Cocci can grow in pairs, chains, or clusters, depending on their orientation and attachment during cell division. In contrast to many bacilli-shaped bacteria, most cocci bacteria do not have flagella and are non-motile.
Cocci is an English loanword of a modern or neo-Latin noun, which in turn stems from the Greek masculine noun κόκκος (cóccos) meaning 'berry'.
Structure[edit]
The cell wall structure for cocci may vary between gram-positive (thick peptidoglycan layers) and gram-negative (thin peptidoglycan layers). While living in their host organism, cocci can be pathogenic (e.g., streptococcus), commensal, or symbiotic.
Gram-positive Cocci[edit]
The gram-positive cocci are a large group of loosely bacteria with similar morphology. All are spherical or nearly so, but they vary considerably in size. Members of some genera are identifiable by the way cells are attached to one another: in pockets, in chains, or grape-like clusters. These arrangements reflect patterns of cell division and that cells stick together. Sarcina cells, for example, are arranged in cubical pockets because cell division alternates regularly among the three perpendicular planes. Streptococcus spp. resemble a string of beads because division always occurs in the same plane. Some of these strings, for example, S. pneumoniae, are only two cells long. They are called diplococci. Species of Staphylococcus have no regular plane of division. They form grape-like structures.
The various gram-positive cocci differ physiologically and by habitat. Micrococcus spp. are obligate aerobes that inhabit human skin. Staphylococcus spp. also inhabit human skin, but they are facultative anaerobes. They ferment sugars, producing lactic acid as an end product. Many of these species produce carotenoid pigments, which color their colonies yellow or orange. Staphylococcus aureus is a major human pathogen. It can infect almost any tissue in the body, frequently the skin. It often causes nosocomial (hospital-acquired) infections.
Arrangements[edit]
Cocci may occur as single cells or remain attached following cell division. Those that remain attached can be classified based on cellular arrangement:
Bacteria are microbes with a cell structure simpler than that of many other organisms. Their control centre, containing the genetic information, is contained in a single loop of DNA. Some bacteria have an extra circle of genetic material called a plasmid rather than a nucleus. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example it may contain a gene that makes the bacterium resistant to a certain antibiotic.
Bacteria are classified into five groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.
Bacteria are found in every habitat on Earth: soil, rock, oceans and even arctic snow. Some live in or on other organisms including plants and animals including humans. There are approximately 10 times as many bacterial cells as human cells in the human body. A lot of these bacterial cells are found lining the digestive system. Some bacteria live in the soil or on dead plant matter where they play an important role in the cycling of nutrients. Some types cause food spoilage and crop damage but others are incredibly useful in the production of fermented foods such as yoghurt and soy sauce. Relatively few bacteria are parasites or pathogens that cause disease in animals and plants.
How do bacteria reproduce?
Most bacteria reproduce by binary fission. In this process the bacterium, which is a single cell, divides into two identical daughter cells. Binary fission begins when the DNA of the bacterium divides into two (replicates). The bacterial cell then elongates and splits into two daughter cells each with identical DNA to the parent cell. Each daughter cell is a clone of the parent cell.
When conditions are favourable such as the right temperature and nutrients are available, some bacteria like Escherichia coli can divide every 20 minutes. This means that in just seven hours one bacterium can generate 2,097,152 bacteria. After one more hour the number of bacteria will have risen to a colossal 16,777,216. That’s why we can quickly become ill when pathogenic microbes invade our bodies.
Survival mechanism
Some bacteria can form endospores. These are dormant structures, which are extremely resistant to hostile physical and chemical conditions such as heat, UV radiation and disinfectants. This makes destroying them very difficult. Many endospore-producing bacteria are nasty pathogens, for example Bacillus anthracis, the cause of anthrax.
Educational resource for students: Observing bacteria cultures in a Petri dish and learning about colony morphology.
The organisms that cause tuberculosis in humans and animals, Mycobacterium tuberculosis and Mycobacterium bovis, are featured in this edition of Microbiology Today alongside Mycobacterium leprae, the cause of leprosy, and Mycobacterium ulcerans, which causes Buruli ulcer.
Often, the first things that come to mind when we think about microbes in the built environment are damage, decay, discolouration and staining to building materials and their surfaces. What we don’t often consider is their ability to act as ‘bioengineers’.
Tuberculosis (TB) is a debilitating multi-organ disease caused by the bacterium Mycobacterium tuberculosis. The most important form of the disease is pulmonary TB, an infection of the lungs and respiratory tract.
The threat of antimicrobial resistance (AMR) has now been recognised globally and it is estimated that 10 million people a year will die due to antimicrobial resistance by 2050 if no urgent action is taken.
Species within the genus Pseudomonas are amongst the most researched bacteria in the scientific community. Bacteria in this genus are widely used as model organisms in microbial research, and include a range of important species in fields such as plant pathogenicity, bioremediation, and environmental microbiology.
As well as being hugely important in the medical and pharmaceutical industries, Streptomyces also play a significant environmental role; contributing to the decomposition of organic matter, and fertility of the soil.
What shape of bacteria is round clusters?
coccus, plural Cocci, in microbiology, a spherical-shaped bacterium.Which of the following bacteria are clusters of circular bacteria?
Coccus indicates round or oval in shape. Therefore, Staphylococci are round or oval-shaped organisms arranged in a cluster.What type of bacteria grows in clusters?
Round bacteria that grow in clusters are called staphylococcus. The round-shaped bacteria (coccus) are called Staphylococcus. Examples of bacteria of the genus Staphylococcus include Staphylococcus aureus and Staphylococcus haemolyticus.Which round bacteria grow in pairs?
Coccus refers to the shape of the bacteria, and can contain multiple genera, such as staphylococci or streptococci. Cocci can grow in pairs, chains, or clusters, depending on their orientation and attachment during cell division.