Streptococcus pneumoniae, or pneumococcus, is a Gram-positive, alpha-hemolytic (under aerobic conditions) or beta-hemolytic (under anaerobic conditions), facultative anaerobic member of the genus Streptococcus. They are usually found in pairs (diplococci) and do not form spores and are non-motile. As a significant human pathogenic bacterium S. pneumoniae was recognized as a major cause of pneumonia in the late 19th century, and is the subject of many humoral immunity studies.
S. pneumoniae resides asymptomatically in healthy carriers typically colonizing the respiratory tract, sinuses, and nasal cavity. However, in susceptible individuals with weaker immune systems, such as the elderly and young children, the bacterium may become pathogenic and spread to other locations to cause disease.It spreads by direct person-to-person contact via respiratory droplets and by autoinoculation in persons carrying the bacteria in their upper respiratory tract. It can be a cause of neonatal infections.
S. pneumoniae is the main cause of community acquired pneumonia and meningitis in children and the elderly, and of septicemia in those infected with HIV. The organism also causes many types of pneumococcal infections other than pneumonia. These invasive pneumococcal diseases include bronchitis, rhinitis, acute sinusitis, otitis media, conjunctivitis, meningitis, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess.
S. pneumoniae can be differentiated from the viridans streptococci, some of which are also alpha-hemolytic, using an optochin test, as S. pneumoniae is optochin-sensitive. S. pneumoniae can also be distinguished based on its sensitivity to lysis by bile, the so-called "bile solubility test". The encapsulated, Gram-positive coccoid bacteria have a distinctive morphology on Gram stain, lancet-shaped diplococci. They have a polysaccharide capsule that acts as a virulence factor for the organism; more than 90 different serotypes are known, and these types differ in virulence, prevalence, and extent of drug resistance.
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History
In 1881, the organism, known later in 1886 as the pneumococcus for its role as a cause of pneumonia, was first isolated simultaneously and independently by the U.S. Army physician George Sternberg and the French chemist Louis Pasteur.
The organism was termed Diplococcus pneumoniae from 1920 because of its characteristic appearance in Gram-stained sputum. It was renamed Streptococcus pneumoniae in 1974 because it was very similar to streptococci.
S. pneumoniae played a central role in demonstrating that genetic material consists of DNA. In 1928 Frederick Griffith demonstrated transformation of life turning harmless pneumococcus into a lethal form by co-inoculating the live pneumococci into a mouse along with heat-killed virulent pneumococci. In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty demonstrated the transforming factor in Griffith's experiment was not protein, as was widely believed at the time, but DNA. Avery's work marked the birth of the molecular era of genetics.
Genetics
The genome of S. pneumoniae is a closed, circular DNA structure that contains between 2.0 and 2.1 million base pairs depending on the strain. It has a core set of 1553 genes, plus 154 genes in its virulome, which contribute to virulence and 176 genes that maintain a noninvasive phenotype. Genetic information can vary up to 10% between strains.
Transformation
Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the surrounding medium. Transformation is a complex developmental process requiring energy and is dependent on expression of numerous genes. In S. pneumoniae at least 23 genes are required for transformation. In order for a bacterium to bind, take up and recombine exogenous DNA into its chromosome it must enter a special physiological state called competence.
Competence in S. pneumoniae is induced by DNA-damaging agents such as mitomycin C, fluoroquinolone antibiotics (norfloxacin, levofloxacin and moxifloxacin), and topoisomerase inhibitors. Transformation protects S. pneumoniae against the bactericidal effect of mitomycin C. Michod et al. summarized evidence that induction of competence in S. pneumoniae is associated with increased resistance to oxidative stress and increased expression of the RecA protein, a key component of the recombinational repair machinery for removing DNA damages. On the basis of these findings they suggested that transformation is an adaptation for repairing oxidative DNA damages. S. pneumoniae infection stimulates polymorphonuclear leukocytes (granulocytes) to produce an oxidative burst that is potentially lethal to the bacteria. The ability of S. pneumoniae to repair the oxidative DNA damages in its genome, caused by this host defense, likely contributes to this pathogen's virulence. Consistent with this premise Li et al. reported that, among different highly transformable S. pneumoniae isolates, nasal colonization fitness and virulence (lung infectivity) depend on an intact competence system.
Infection
S. pneumoniae is part of the normal upper respiratory tract flora. As with many natural flora it can become pathogenic under the right conditions, typically when the immune system of the host is suppressed. Invasins, such as pneumolysin, an anti-phagocytic capsule, various adhesins, and immunogenic cell wall components are all major virulence factors. After S. pneumoniae colonizes the air sacs of the lungs, the body responds by stimulating the inflammatory response causing plasma, blood, and white blood cells to fill the alveoli. This condition is called pneumonia. It is susceptible to clindamycin.
Diseases and Symptoms
Pneumonia is the most common and most serious of the S. pneumoniae diseases which include symptoms like fever and chills, cough, rapid breathing, difficulty breathing, chest pain. For the elderly they may include confusion, low alertness, and less of the former listed symptoms.
Pneumococcal meningitis is an infection of the tissue covering the brain and spinal cord. Symptoms include: stiff neck, fever, headache, confusion and photophobia.
Sepsis is caused by overwhleming response to an infection and leads to tissue damage, organ failure, and even death. The symptoms include: confusion, shortness of breath, elevated heart rate, pain or discomfort, over-perspiration, fever, shivering, or feeling cold.
Vaccine
Due to the importance of disease caused by S. pneumoniae several vaccines have been developed to protect against invasive infection. The World Health Organization recommend routine childhood pneumococcal vaccination; it is incorporated into the childhood immunization schedule in a number of countries including the United Kingdom, United States, and South Africa.
Interaction with Haemophilus influenzae
Historically Haemophilus influenzae (H. influenzae) has been a significant cause of infection and both H. influenzae and S. pneumoniae can be found in the human upper respiratory system. A study of competition in vitro revealed S. pneumoniae overpowered H. influenzae by attacking it with hydrogen peroxide. However, in a study adding both bacteria to the nasal cavity of a mouse within 2 weeks only H. influenzae survives; further analysis showed that neutrophils exposed to dead H. influenzae were more aggressive in attacking S. pneumoniae.
Diagnosis
Diagnosis is generally made based on clinical suspicion along with a positive culture from a sample from virtually any place in the body. An ASO Titre of >200 units is significant. S. pneumoniae is, in general, optochin sensitive, although optochin resistance has been observed.
The recent advances in next-generation sequencing (NGS) and comparative genomics have enabled the development of robust and reliable molecular methods for the detection and identification of S. pneumoniae. For instance, the "Xisco" gene was recently described as a biomarker for PCR-based detection of S. pneumoniae and differentiation from closely-related species.
Atromentin and leucomelone possess antibacterial activity, inhibiting the enzyme enoyl-acyl carrier protein reductase, (essential for the biosynthesis of fatty acids) in S. pneumoniae. Optochin sensitivity in a culture of Streptococcus pneumoniae
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