Gram Positive Cocci and Streptococci PDF

Gram positive cocci Staphylococcus Properties : 1- Staphylococcus is Normal flora of the human skin and mucous membranes. Most of Staphylococcus spp. are human commensals, like Staph. epidermidis but others may be pathogens such as : Staph. aureus 2- gram-p...

Gram positive cocci Staphylococcus Properties : 1- Staphylococcus is Normal flora of the human skin and mucous membranes. Most of Staphylococcus spp. are human commensals, like Staph. epidermidis but others may be pathogens such as : Staph. aureus 2- gram-positive grape cluster like cocci. 3- non spore forming , non motile 4- facultative anaerobe whose growth is enhanced in the presence of O2 and CO2. Staphylococcus aureus characteristics : 1- The optimum temp. of growth is 37°C. On blood agar colonies varies from golden to white with β- hemolysis. 2- This sp. can resist high salt concentrations ( 7.5 – 10% ) , it is so called Halophilic bacteria And resist extreme pH, and high temperatures ( up to 60ᴼC for 60 minutes ). 3- It also remains viable after months of air drying and resists the effects of many disinfectants and antibiotics. Some Enzymes of S. aureus 1- Catalase which converts H2O2 to water and oxygen. 2- Coagulase an enzyme that cause clot of plasma. 3- Penicillinase analyzes penicillin. 4- hyaluronidase lysis hyaluronic acid Toxins of S. aureus 1-Hemolysins lyse red blood cells by disrupting their membranes, and produce a zone of hemolysis in blood agar. 2- Leucocidin which damages the neutrophils and macrophages. 3- Enterotoxins that causes food poisoning. Types of Staphylococcus aureus infections 1- skin infections ( pustules , impetigo , Staphylococcal scalded skin syndrome (SSSS) , wound and burn infections. 2- conjunctivitis 3- food poisoning 4- septicemia Genus Streptococcus 1- This genus includes a large and varied group of Spp.. Some are normal residents and some are pathogenic to humans and animals. 2- gram +ve cocci arranged in different length chains. 3- Streptococci are non-spore-forming and non motile ,. 4- They are facultative anaerobes that , Colonies are usually small. Most parasitic forms are fastidious in nutrition and require enriched media for cultivation. 5- Most members of the genus are quite sensitive to drying , heat , and disinfectants. Despite the large number of streptococcal species , human disease is most often associated with S. pyogenes , S. pneumoniae , and Enterococcus faecalis. Classification of Streptococci : Streptococci can be classified according to : 1- Hemolytic properties on Blood agar : 1- β-Hemolytic Streptococci : Causes complete lysis of red blood cells resulting in a clear ring around the colony for example Streptococcus pyogenes. 2- α-Hemolytic Streptococci : Causes chemical changes in the hemoglobin of RBCs in blood agar resulting in the appearance of green color that forms a ring around the colony for ex. Streptococcus pneumonia. 3- γ-Hemolytic Streptococci don’t lyse RBCs. E.g. Streptococcus mutans 2- Surface antigenic properties According to the surface antigens ; Lancefield scheme classifies Streptococci into groups from A to U. The most important clinically groups of β-hemolytic Streptococci are type A and B. - Strep. pyogenes ( group A ) - Strep. agalactiae ( group B ) - Enterococcus ( group D ) Enzymes and Toxins of Streptococcus : a- ( hemolysin ) streptolysin O and streptolysin S lysis RBCs b- streptokinase lysis fibrin c- hyaluronidase lysis hyaluronic acid d- leucocidin lysis WBCs Diseases caused by S. pyogenes 1- tonsillitis 2- Rheumatic fever 3- Burn and Wound infections Genus Enterococcus 1- Enterococcus spp. are facultatively anaerobic, catalase-negative , Gram- positive cocci, arranged individually, in pairs, or short chains , they are called the enterococci because they are normal flora of the large intestine and causes opportunistic infections of wounds , blood , urinary and gastrointestinal tracts.  Compared with other clinically important gram-positive cocci, Enterococcus (especially E. faecium and E. faecalis) is intrinsically more resistant to the antimicrobial agents commonly used in acute and long-term health care settings. Like Vancomycin ; an antibiotic used to treat infections caused by gram-positive bacteria ; it is so called vancomycin-resistant Enterococcus (VRE).