Gram-Positive Cocci PDF

Chapter 3 Gram-Positive Cocci Part 3-A Prepared by: Dr. Muna M. Abbas 1  Gram-Positive Cocci Staphylococci and Streptococci Single cell: Coccus (spherical), Gram-positive Cellular...

Chapter 3 Gram-Positive Cocci Part 3-A Prepared by: Dr. Muna M. Abbas 1  Gram-Positive Cocci Staphylococci and Streptococci Single cell: Coccus (spherical), Gram-positive Cellular arrangement: 1- Staphylococci: Grape-like clusters 2- Streptococci : Chains (two or more) Staphylococci: General features: Spherical Gram-positive cocci that are arranged in grape-like clusters Catalase positive Non motile Facultative anaerobic bacteria Medically important species of Staphylococci are: Staphylococcus aureus (most important) Stphylococcus epidermidis Stphylococcus saprophyticus Based on the production of Coagulase enzyme, Staphylococci can be classified into: 1- Coagulase positive-Staphylococci (Produce Coagulase enzyme) Example: Staphylococcus aureus 2- Coagulase negative Staphylococci (Do not produce Coagulase enzyme) Examples: Staphylococcus epidermidis and Staphylococcus saprophyticus). Note: Coagulase is an enzyme induces coagulation of plasma by activating prothrombin to form thrombin. Thrombin in turns catalyzes the activation of fibrinogen to form fibrin, which causes clot formation Staphylococcus aureus: Epidemiology and Transmission: Approximately 30-50% of people harbor Staphylococcus aureus in their anterior nares. It is also found in the vagina of approximately 5% of women From the anterior nares, Staphylococcus aureus may spread to the skin and throat Staphylococcus aureus may be found almost every where around us ( tables, bed, clothes…..) Hand contact is an important mode of transmission Golden-yellow colonies. MRSA strains are antibiotic-resistant MRSA are usually treated with Vancomycin, but VRSA appeared Staphylococcus aureus: Most Important virulence factors that can be found in Staph aureus: 1- Teichoic acid: involved in attachment and may cause septic shock 2- Collagen-binding proteins: involved in attachment to collagen and explain why this bacterium is common in causing wound infection 3- Exotoxins: Connective tissue degrading enzymes such (proteases such as collagenase and elastase), lipases, hyaluronidase. Pore-forming toxins such as PV Leukocidin (inactivate and kill leukocytes) Toxic Shock Syndrome Toxin (TSST): a super-antigen that mediate non-specific activation of T cells and consequently leads for the production of massive amounts of cytokines. Exfoliative toxin: involved in scalded skin syndrome Enterotoxin B: it is heat stable and it is involved in food poisoning Staph aureus infections: A- Local skin infections: A local inflammation that is characterized by tissue destruction and pus formation and accumulation such as folliculitis, boils and wound infection. When these infections become massive, they lead for the formation of abscess (furuncles, carbuncles) formation that can be associated with high fever (pyogenic inflammation) B-Systemic infections: From major abscesses, S aureus reaches the blood stream (septicemia that can be associated with septic shock ) and thus can get seeded in different tissues and organs to cause internal infections (abscesses) in these locations. Examples: endocarditis, osteomyelitis, meningitis, pneumonia. Note: septic shock can be induced by Teichoic acid and/or TSST Symptoms of septic shock: Initially: High fever and Tachycardia Later on: decrease in body temperature, disseminated intravascular thrombosis , bleedings, sudden drop in blood and circulatory collapse, Multiple organ failure, comma, cardiac arrest and death Other diseases caused by S aureus: Non-pyogenic skin infection such cellulites and impetigo Scalded-skin syndrome: (occurs most frequently in children) Some strains of S aureus produce epidermolytic toxins known as Exfoliative toxins, which are proteases that specifically cleave desmogleins of superficial layers of the skin. This results in a clinical condition known as Scalded Skin Syndrome (Ritter’s Syndrome) that is characterized by fever, large areas of skin slough, secondary skin infections and electrolyte imbalance can occur. Food poisoning: Staphylococcal Food Poisoning (Staphylococcal Enterotoxicosis) Some strains of Staph aureus produce a heat-stable and an acid-stable exotoxin known as Enterotoxin B. Ingestion of food heavily contaminated with such kinds of S aureus strains result in a self- limiting vomiting that could be associated with watery diarrhea Enterotoxin B is a super-antigen induces production of cytokines from mesenteric lymph nodes after ingestion of food contaminated food. These cytokines stimulate the intestinal nervous system, which in turns, stimulate the vomiting center in the brain. Staphylococcus epidermidis and Staphylococcus saprophyticus Both are two coagulase-negative Staphylococci. Staphylococcus epidermidis: It is part of the normal human flora on the skin and mucous membranes (Ninety percent of normal skin microbiota). It commonly infects intravenous catheters and prosthetic implants, e.g., prosthetic heart valves (endocarditis), vascular grafts, and prosthetic joints (arthritis or osteomyelitis) It is also a major cause of sepsis in neonates and of peritonitis in patients with renal failure who are undergoing peritoneal dialysis through an indwelling catheter. Staphylococcus saprophyticus: It causes urinary tract infections, particularly in sexually-active young women. Figure 21.3 Coagulase-Negative Staphylococci Biofilm Surface of catheter Biofilm Catheter sur- Most of the face with adhering bacteria produc- bacteria. Biofilm, light green, is ing the slime are not visible beginning to appear. under the biofilm. Copyright © 2021 Pearson Education Ltd. All Rights Reserved Streptococci: Streptococci are spherical Gram-positive cocci that arranged in chains. The shortest chain of Streptococci consists of a pair of cocci (diplococci). All streptococci are: ❑Catalase-negative (Remember, Staphylococci are Catalase-positive) ❑Non-motile ❑Facultative anaerobes Classification of Streptococci: I- According to their hemolytic activities: Based on the hemolytic activities of the colonies of different Streptococcal species seen on Blood agar, Streptococci are classified as: 1-Alpha Hemolytic Streptococci: form a green zone around their colonies due to hemoglobin reduction to methemoglobin by hydrogen peroxide produced by the bacterium 2- Beta-Hemolytic Streptococci: form a clear zone around their colonies due to complete lysis of the red cells occurs. (These species of Streptococci produce pore-forming exotoxins known as hemolysins. Hemolysins form pores in the membrane of red blood cells and cause their hemolysis (hence the name hemolysin). However, it has been found that these toxins can form pores in the membrane of other type of cells as well. 3-Gamma Hemolytic Streptococci: These are non-hemolytic species of Streptococci (no change is seen o blood agar around their colonies)  II- According to Lance-field Grouping: Lance-field Grouping is based on a particular CHO antigen of the wall of MOST Beta hemolytic Streptococci, known as C carbohydrate. According to this classification, there are currently 18 groups, ( A, B, C D…….) Most Streptococcal infections are caused by members of Streptococci that are part of the human microbiota. Accordingly, most Streptococcal infections are endogenous infections. Examples: Viridans streptococci and Streptococcus pneumoniae are found mainly in the oropharynx Streptococcus agalactiae occurs in the vagina and colon Enterococci and anaerobic Streptococci are located in the colon Streptococcus pyogenes (Beta-hemolytic/Group A) It is the most important streptococcal pathogen. Some times, it be found transiently on skin and throat (it can be considered as a primary pathogen) Epidemiology and Transmission of Streptococcus pyogenes: Epidemiology: Streptococcus pyogenes is the most common cause of throat infection (Pharyngitis) , specially among children. Transmission: Respiratory aerosols Diseases: 1- Pharyngitis: Throat infection with Strep pyogenes can lead pyogenic inflammation (Pharyngitis/Sore throat/Strep throat). Sore throat is characterized by a painful, swollen and inflamed tonsils with the formation of exudates, high fever, leukocytosis, and tender cervical lymph nodes. Pharyngitis caused by some strains of Strep pyogenes that produces an exotoxin known as an Erythrogenic toxin, can be associated with a distinctive pink-red sandy skin rash and strawberry-colored tongue , a clinical condition that is known as Scarlet Fever. Note: Step pyogenes produces an enzyme known as Streptokinase (fibrinolysin) that dissolve fibrin. Accordingly, it was possible to use this enzyme to dissolve fibrin in clots, thrombi, and emboli, Streptokinase is used to lyse thrombi in the coronary arteries of heart attack patients. 2-Otitis media 3- Bronchitis and pneumonia 4- Bacteremia and Toxic Shock: bacteremia caused by Strep pyogenes can be associated with septic shock. Massive production of cytokines from T cells can be induced by its Pyogenic Exotoxin A as well as its teichoic acid. 5- Meningitis Skin infections caused by Step pyogenes: Step pyogenes can cause non-pyogenic skin infections such as cellulites and impetigo Strep Gangrene (necrotizing fasciitis) or “Flesh-eating” disease: skin infection caused by certain strains of Streptococcus pyogenes that produces a particular exotoxin known as Exotoxin B (a Cysteine protease) can cause severe necrotizing fasciitis that is characterized by massive tissue necrosis. As the tissue dies, Streptococcus pyogenes enter the blood and rapidly spreads throughout the body causing sepsis and septic shock. This may lead to death. Figure 21.8 Necrotizing Fasciitis of a Leg Due to Group A Streptococci Copyright © 2021 Pearson Education Ltd. All Rights Reserved Erysipelas S. pyogenes infects the dermal layer of the skin. Causes local tissue destruction and sepsis Figure 21.7 Lesions of Erysipelas, Caused by Group A Beta- hemolytic Streptococcal Toxins Puerperal Sepsis Also called puerperal fever and childbirth fever – Caused by Streptococcus pyogenes – Transmitted to the mother during childbirth ▪ Infects the uterus and progresses to an infection of the abdominal cavity (peritonitis) Copyright © 2021 Pearson Education Ltd. All Rights Reserved Post-Streptococcal Diseases: Some strains of Streptococcus pyogenes have certain M proteins with antigenic properties that is similar to certain antigens in human body that induce the formation of cross-reacting auto-antibodies. As these cross-reacting antibodies bind to human antigens, to cause serious clinical conditions such as: 1- Acute Glomerulonephritis: which may lead to compete renal failure 2- Acute rheumatic fever: characterized by myocardial and endocardial tissue damage, especially in heart valves. Acute Glomerulonephritis Is an inflammatory condition that affects the glomeruli, which are the small filtering units in the kidneys. It leads to a disruption of the kidneys' ability to filter blood properly, and it can result in a variety of symptoms, including blood and protein in the urine, swelling, and high blood pressure. It is caused by hemolytic streptococcus group A, so-called nephritogenic M strains (most commonly types 12 and 49). The disease develops 1-3 weeks after recovery from a streptococcal infection (tonsillitis, pharyngitis, sinusitis, otitis media, scarlet fever, impetigo contagiosa streptogenes, streptococcal foci). Nephritogenic streptococci produce antigens, the most important of which are NSAP- streptokinase (nephritogenic strains associated protein), M-protein, and the so-called endostreptosin. These antigens are then followed by specific antibodies which, after binding with the antigen, form deposits of immunocomplexes on basal membranes of glomerular capillaries. Copyright © 2021 Pearson Education Ltd. All Rights Reserved Rheumatic Fever Autoimmune complication of S. pyogenes infections Inflammation of the heart valves – Immune reaction against streptococcal M protein Subcutaneous nodules at the joints Sydenham’s chorea – Purposeless, involuntary movements Copyright © 2021 Pearson Education Ltd. All Rights Reserved Figure 23.5 A Nodule Caused by Rheumatic Fever Nodule Elbow joint Copyright © 2021 Pearson Education Ltd. All Rights Reserved