DIPLOCOCCI: CHARACTERISTICS, MORPHOLOGY, TYPES, EXAMPLES - BIOLOGY - 2025

The diplococci are bacteria spherical or ovoid normally held together in pairs after cell division. They include some pathogenic species of great medical and veterinary importance, as well as numerous much less studied free-living species.

Diplococci are not a monophyletic group, that is, evolutionarily they do not come from an exclusive common ancestor. Therefore, they do not receive a scientific name that is used for all of them together.

Source: Photo Credit: Content Providers (s): CDC / Dr. Norman Jacobs

Among the human diseases caused by bacteria classified as diplococci are arthritis, bronchitis, cellulitis, conjunctivitis, erysipelas and other skin conditions, necrotizing fasciitis, puerperal fever, gangrene, respiratory infections (pneumonia and others), meningitis, myocarditis, otitis media, septicemia, sinusitis, and non-gonococcal urethritis (gonorrhea).

Characteristics and morphology

Based on their shape, bacteria can be classified into three main types:

- Spherical or ovoid (cocci)

- Cylindrical rods (bacilli)

- Curved or helical swabs (spirillae and spirochetes).

Also distinguished are: short rods (coccobacilli); curved rods (vibrios); cells of indefinite or variable shape (pleomorphic bacteria).

After cell division, cocci can appear as isolated bacteria, or as pairs or groups of bacteria united. In the latter case, they can be classified, according to their way of grouping, into even cocci (diplococci), chain cocci (streptococci), or cluster cocci (staphylococci).

The appearance of diplococci and staphylococci is caused by cell division in one plane. The appearance of streptococci is caused by cell division in multiple planes.

Diplococci, staphylococci, and streptococci are slightly flattened on their adjacent surfaces. Thus, in the case of diplococci, they are often said to have the appearance of joined coffee beans (just as these beans are found inside the fruit).

Evidence that in bacteria the grouped arrangement of cells can be convergent, there are also diplobacilli, which are bacilli in which, in a similar way to diplococci, the two cells produced by the same cell division remain united.

History of the term diplococcus

In 1881, G. Sternberg discovered the pneumococcus, naming it "Micrococcus Pasteuri." That same year, L. Pasteur also identified it, referring to it as the “microbe septicèmique du salive”.

At present, the genus Micrococcus (Cohn, 1872) is still valid, but it is used for other species of bacteria not closely related to pneumococcus. Similarly, the specific epithet pasteuri applies to other species of bacteria.

In 1886, A. Weichselbaum coined the name Diplococcus pneumoniae for pneumococcus. However, it was not until 1920 that this scientific name was accepted by the Society of American Bacteriologists.

Based on its characteristic of forming chain cocci when cultivated in liquid medium, in 1974, the pneumococcus was reclassified into the genus Streptococcus. Since then it has been known as Streptococcus pneumoniae. Molecular phylogenies support the pneumococcus belonging to the genus Streptococcus, which includes other pathogenic species.

Diplococcus is no longer used as a scientific name. The informal term "diplococcus" subsists to refer to bacteria of diverse evolutionary origins and different biologies that have in common the characteristic of grouping in pairs.

ID

Diplococcal status is an uninteresting feature that, like other distinguishing features, is useful in identifying certain species of disease-causing bacteria.

The first step in the identification of bacteria is to determine the morphology, and whether or not there is an aggregated arrangement of their cells. One result of that first step may be to establish that the bacteria are diplococci. However, the final identification in most cases depends on additional phenotypic and genotypic characteristics.

Pneumonia with purulent sputum can be caused by a diplococcus (Streptococcus pneumoniae). However, the oral bacterial flora contains other diplococci. The characteristic of being diplococcus is insufficient to identify S. pneumoniae.

In men, the presence of Gram negative cocci in urethral secretions can diagnose gonorrhea. However, in women, the cervix may contain Gram-negative cocci that do not cause gonorrhea, so it is necessary to consider other bacterial characteristics to diagnose the disease.

Coccobacilli of the genus Acinetobacter take on the appearance of diplococci in body fluids and culture media. Because of this appearance, they can be confused with the Neisseria species that cause gonorrhea, meningitis, and septicemia. This problem is avoided considering that Acinetobacter does not produce oxidases and Neisseria does.

Types

Diplococci can be either Gram positive or Gram negative. The former acquire an intense blue color when they retain the violet stain of the Gram stain. The latter acquire a light pink color by not retaining said dye.

The Gram stain is a procedure devised in 1844 by the Danish physician Hans Christian Gram (1853–1938) that reveals differences between bacteria in terms of the structural and biochemical properties of their cell walls. These properties help group bacteria into different taxonomic and functional categories.

The retention of the blue color is due to the fact that the bacteria have a thick cell wall that prevents the penetration of solvents. The acquisition of a pink color is due to the fact that the bacteria have a thin cell wall that allows the solvent to penetrate and remove the violet dye. The Gram stain is another initial step in identifying bacteria.

Gram-positive bacteria are more susceptible than Gram-negative bacteria to penicillins, acids, iodine, and basic compounds, which obviously has therapeutic implications.

Examples

Moraxella catarrhalis

Moraxella (Branhamella) catarrhalis. It is a Gram negative coconut. It was formerly called Micrococcus catarrhalis, or Neisseria catarrhalis. Up to 75% of children are carriers. In contrast, only 1–3% of healthy adults are.

Before the 1970s, it was considered a commensal bacteria of the upper respiratory tract. Subsequently, it has been considered a common and important pathogen of said tract.

Taxonomically, it belongs to the Moraxellaceae family, of the Pseudomonadales order, of the Gammaproteobacteria class, of the Chlorobi phylum, of the Bacteria domain.

Neisseria gonorrhoeae

Neisseria gonorrhoeae and N. meningitides. They are Gram negative cocci. Humans are the only known reservoirs.

N. gonorrhoeae is a pathogenic species in 100% of cases. In the case of N. meningitidis, ~ 20% of the population carry it in the throat. Half of the strains of N. meningitidis are noncapsulated, therefore non-pathogenic.

Taxonomically, they belong to the Neisseriaceae family, of the Neisseriales order, of the Betaproteobacteria class, of the Chlorobi phylum, of the Bacteria domain.

Streptococcus pneumoniae

It is a Gram positive coconut, sometimes forming short rods. It is one of the best scientifically studied living beings. It is a normal inhabitant of the nasopharynx in 5–10% of adults and 20–40% of children. It is a very important pathogen, representing the most frequent cause of lobar pneumonia.

The characteristics of S. pneumoniae allow it to colonize various niches. From the nasopharynx they can pass into the lower respiratory tract, causing pneumococcal lobal pneumonia. This colonization in turn can be the focus of an invasion (bacteremia, septicemia) of the blood, from which it can pass to the meninges (meningitis).

Taxonomically, it belongs to the Streptococcaceae family, of the Lactobacillales order, of the Bacilli class, of the Firmicutes phylum, of the Bacteria domain.

Pathologies

Moraxella catarrhalis

Most often in children and the elderly. Moraxella catarrhalis causes eye infections, otitis media, sinusitis, tracheitis, bronchitis, pneumonia, and bacteremia. In adults, it causes chronic obstructive pulmonary disease.

In immunosuppressed patients, it causes meningitis, endocarditis, and septicemia. In children and adults, it is the cause of hospital respiratory infections.

Sinusitis is a very common infection in young children. M. catarrhalis is the cause of approximately 20% of cases. Acute otitis media and lower respiratory tract infections are also common in children, particularly those under three years of age.

Neisseria gonorrhoeae and N. meningitides

Neisseria gonorrhoeae (gonococcus) causes gonorrhea, which manifests itself primarily as a copious discharge of purulent secretions from the male and female urethra and the female cervix. Secondary local complications are common, such as epididymitis, salpingitis, rectal infection, pharyngeal infection, and pelvic inflammatory disease.

In the absence of adequate treatment, skin lesions, arthritis, ophthalmia, liver inflammation, endocarditis, meningitis, and fever may occur.

Neisseria meningitidis (meningococcus) is the only bacterium capable of producing outbreaks of pyogenic meningitis. These outbreaks require transmission between the nasopharynxes of nearby people, either by direct physical contact, or by droplets of mucus that travel through the air. In third world countries, ~ 10% of cases are fatal.

Meningococci can also cause conjunctivitis, endocarditis, sore throat, meningitis, meningoencephalitis, myocarditis, pericarditis, peritonitis, and acute septicemia.

Streptococcus pneumoniae

The natural habitat of Streptococcus pneumoniae (pneumococcus) is the nasopharynx, especially that of children.

Infections caused by S. pneumoniae fall into two categories: 1) invasion of the skin and mucosa, such as sinusitis, otitis media, and conjunctivitis; 2) invasive infections, such as bronchitis, pneumonia, bacteremia, meningitis, endocarditis, septic arthritis and meningitis.

S. pneumoniae and N. meningitidis are the main causes of bacterial meningitis, which usually cause fever, migraine, and stiff neck.

In the pre-antibiotic era, pneumonia caused by S. pneumoniae was common and fatal. This pneumonia remains one of the most important factors of mortality in African children.

The great epidemiological importance and danger of this pneumonia has determined that pneumococcal vaccines have been developed.

References

1. Alcamo, IE 1996. Cliffs quick review: microbiology. Wiley, New York.
2. Basualdo, JA, Coto, CE, de Torres, RA 2006. Medical microbiology. Editorial Atlante, Buenos Aires.
3. Bauman, RW 2012. Microbiology: with diseases by body system. Benjamin Cummings, Boston.
4. Bottone, EJ 2004. An atlas of the clinical microbiology of infectious diseases, Volume 1, bacterial agents. Parthenon, Boca Raton.
5. Brooks, GF, Butel, JS, Carroll, KC, Morse, SA 2007. Jewetz, Melnick & Adelberg᾿s medical microbiology. McGraw-Hill, New York.
6. Cimolai, N. 2001. Laboratory diagnosis of bacterial infections. Marcel Dekker, New York. Principles and practice of clinical bacteriology
7. Garrity, GM, Brenner, DJ, Krieg, NR, Staley, JT 2006. Bergey's manual ^® of systematic bacteriology, Second Edition, Volume Two, The Proteobacteria, Part A, Introductory essays. Springer, Cham.
8. Gillespie, SH, Hawkey, PM 2006. Principles and practice of clinical bacteriology. Wiley, Chichester.
9. Holmes, KK, Sparling, PF, Stamm, WE, Piot, P., Wasserheit, JN, Corey, L., Cohen, MS, Watts, DH 2008. Sexually transmitted diseases. McGraw-Hill, New York.
10. Leboffe, MJ, Pierce, BE 2011. A photographic atlas for the microbiology laboratory. Morton, Englewood.
11. Levinson. W. 2016. Review of medical microbiology and immunology. McGraw-Hill, New York.
12. Sternberg, GM 1886. On Micrococcus pasteuri (Sternberg). Journal of the Royal Microscopical Society, 6, 391–396.
13. Talaro, KP, Talaro, A. 2002. Foundations in microbiology. McGraw-Hill, New York.
14. Török, E., Moran, E., Cooke, FJ 2017. Oxford handbook of infectious diseases and microbiology. Oxford University Press, Oxford.
15. Tortora, GJ, Funke, BR, Case, CL 2010. Microbiology: an introduction. Benjamin Cummings, San Francisco.
16. Watson, DA, Musher, DM, Jacobson, JW, Verhoef, J. 1993. A brief history of the pneumococcus in biomedical research: a panoply of scientific discovery. Clinical Infectious Diseases, 17, 913–924.