under the microscope
By Rodney E . Rohde , PhD , MS , SM ( ASCP ) CM SVCM , MBCM , FACSc
A Visit From an Old Nemesis , Streptococcus
Recently , a group of infectious disease scientists at Houston
Methodist Hospital identified strains of group A streptococcus that are less susceptible to commonly used antibiotics like penicillin and other related beta-lactams . While this genus of bacteria has not been seen as worrisome in regards to antibiotic resistance , these findings remind us that the agent of strep throat , flesh eating disease , rheumatic fever , glomerulonephritis and other dangerous illnesses must never be overlooked or neglected when it comes to research , diagnosis and treatment . James M . Musser , MD , PhD , lead author of the study and chair of the Department of Pathology and Genomic Medicine at Houston Methodist Hospital and members of his department collaborated with nearly a dozen institutions across seven countries ; discuss this research study in the Jan . 29 , 2020 online issue of the Journal of Clinical Microbiology .
Streptococcus is a genus of gram-positive coccus ( plural cocci ) or spherical bacteria that belongs to the family Streptococcaceae , within the order Lactobacillales ( lactic acid bacteria ), in the phylum Firmicutes . The streptococcus bacteria are found in the form of twisted chains (“ strepto ”) of coccus ( spheres , circular body ). At present , there are over 50 medically significant species of this genus . Most streptococci are oxidase-negative and catalase-negative , and many are facultative anaerobes , which means that they are capable of growing both aerobically and anaerobically . In 1984 , many bacteria formerly grouped in the genus Streptococcus were separated out into the genera Enterococcus and Lactococcus .
Many of the species found in the genus are known to be a part of the respiratory and salivary microbiome . Some species can survive on a dry surface for three days to six months . Here , I will focus on the streptococci of medical and clinical importance aimed at a general understanding of the characteristics of this pathogen in the environment . Primarily , I will utilize information obtained from the Centers for Disease Control and Prevention ( CDC ) along with professional experience .
How does transmission occur with these bacteria ? These bacteria are transmitted primarily by direct contact with secretions from oral and nasal discharges of infected people or by coming into contact with wounds ( sores ) on the skin . Risk of transmission to a new host is highest when one comes into contact with an ill person exhibiting these conditions . In the medical setting , the most important groups are the alpha-hemolytic streptococci S . pneumoniae and Streptococcus viridans group , and the beta-hemolytic streptococci of Lancefield groups A and B ( also known as “ group A strep ” and “ group B strep ”).
Which species are of medical significance and what primary diseases do they cause . This group of bacteria are taxonomically classified based on their hemolytic properties ( ability to lyse red blood cells ). The alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells which leads to incomplete or “ partial hemolysis ” causing a greenish color on blood agar . Beta-hemolytic species cause complete lysis of red blood cells appearing as wide areas clear of blood cells surrounding bacterial colonies ( known as zones of
hemolysis ) on blood agar . The gamma-hemolytic species do not cause hemolysis ( non-hemolytic ). Rebecca Lancefield ( scientist ) developed a classification scheme known as the Lancefield groups , a serotype classification ( that is , describing specific carbohydrates present on the bacterial cell wall ). Within the Lancefield grouping , the beta-hemolytic streptococci are named Lancefield group A-W . Species , primary host and their diseases include :
● S . pyogenes ( human ) – pharyngitis , cellulitis , erysipelas [ Group A , beta hemolytic ],
● S . agalactiae ( human , cattle ) – neonatal meningitis and sepsis [ Group B , beta hemolytic ],
● S . dysgalactiae ( human , animals ) – endocarditis , bacteremia , pneumonia , meningitis , respiratory infections [ Group G ],
● S . gallolyticus ( human , animals ) – urinary tract or biliary infections , endocarditis
● body aches [ Group D ],
● S . anginosus ( human , animals ) – subcutaneous / organ abscesses , meningitis , respiratory infections [ viridans streptococci group , alpha hemolytic ],
● S . sanguinis ( human ) – endocarditis , dental caries [ viridans streptococci group , alpha hemolytic ],
● S . mitis ( human ) – endocarditis [ viridans streptococci group , alpha hemolytic ],
● S . mutans ( human ) – dental caries [ viridans streptococci group , alpha hemolytic ], and
● S . pneumoniae ( human ) – pneumonia [ Alpha hemolytic group ].
How can physicians ( clinicians ) and medical laboratory ( clinical microbiologists ) help track this genus ?
Clinicians and microbiologists evaluating pneumococcal or other streptococcus isolates with the following characteristics should contact their state or local health departments for further assistance :
● Pneumococci with potentially novel features , such as an unusual antibiotic susceptibility profiles , and
● Concern about outbreaks related to pneumococci , streptococci ( other than pneumococci ), or other catalase-negative , Gram-positive cocci .
The CDC is available to offer epidemiologic assistance to state and local health departments .
Prevention and treatment .
A single-dose , 23-valent vaccine to prevent infection by the most common serotypes of S . pneumoniae is available in the United States ( see CDC for vaccine recommendations ). Antibiotic ( chemoprophylaxis ) therapy with penicillin for those with rheumatic heart disease from streptococcus may be given monthly ( intramuscular ) or daily ( oral ) for lifetime to prevent development of bacterial endocarditis on a damaged heart valve . Penicillin may also be necessary to control outbreaks of S . pyogenes ( military , nurseries , households , etc .).
Rodney E . Rohde , PhD , MS , SM ( ASCP ) CM SVCM , MBCM , FACSc , serves as chair and professor of the Clinical Laboratory Science Program at Texas State ; associate director for the Translational Health Research Initiative ; as well as associate dean for research in the College of Health Professions . Follow him on Twitter @ RodneyRohde / @ TXST _ CLS , or on his website : http :// rodneyerohde . wp . txstate . edu /