Medical Microbiology I PDF - 2023-2024 - Prof. Dr. Amal Talib Al-Sa'ady

Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady Gram - Positive Cocci / Catalase Positive - Genus: Staphylococcus Depend on Gram staining,...

Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady Gram - Positive Cocci / Catalase Positive - Genus: Staphylococcus Depend on Gram staining, bacteria are classified into two groups: Gram-positive bacteria and gram-negative bacteria. Gram-positive bacteria have a thick cell wall made of peptidoglycan (50–90% of cell envelope), and as a result are stained purple by crystal violet, whereas gram-negative bacteria have a thinner layer (10% of cell envelope), so do not retain the purple stain and are counter-stained pink by safranin. Gram positive cocci are subdivided into two main groups: Staphylococci & Streptococci. Table 1: Taxonomic ranks of Staphylococcus aureus Domain: Bacteria Phylum: Firmicutes Class: Bacilli Order: Lactobacillales Family: Staphylococcaceae Genus: Staphylococcus Species: Staphylococcus aureus Gram - Positive Cocci Catalase Positive - Gram Positive Cocci Genus: Staphylococcus Staphylococci are gram-positive spherical cells, characteristically dividing in more than one plane to form irregular grape-like clusters. Non-motile. Cell wall contains peptidoglycan and teichoic acid. Staphylococcus is catalase-positive and oxidase- negative. Growth Characteristics Staphylococci are Facultative anaerobes. Growth is more rapid and abundant under aerobic conditions. Most strains grow in the presence of 10% Nacl and between 18- 40°C. Staphylococci are relatively resistant to drying, heat )they withstand 50°C for 30 minutes), and 9% sodium chloride but are readily inhibited by certain chemicals (eg, 3% hexachlorophene). Susceptible to lysis by lysostaphin, but resistant to lysis by lysozyme. Natural populations are mainly associated with skin, skin glands, and mucous membranes. Some species may be isolated from a variety of animal products(e.g., meat, milk, cheese) and environmental sources (e.g., fomites, soil, sand, dust, air, or natural waters). Some species are opportunistic pathogens of humans 1  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady and/or animals. They grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow. Classification of Staphylococci Depend on coagulase test, Staphylococci classify into two groups: 1 Coagulase-Positive Staphylococci 2 Coagulase-Negative Staphylococci(CoNS) 1 Coagulase-Positive Staphylococci: Staphylococcus aureus This group has an ability to produce the coagulase enzyme. The most frequently encountered species of clinical importance for this group is Staphylococcus aureus. Almost every person will have some type of its infection during a lifetime. S. aureus has long been recognized as one of the major human pathogens and is by far one of the most common nosocomial bacteria with it' s ability to cause large spectrum of human diseases, ranging from skin lesions to invasive and to severe life- threatening infections (It will be explained later). S. aureus and S. intermedius are coagulase positive. All other staphylococci are coagulase negative. Methicillin - Resistant S. aureus (MRSA): Methicillin(Meticillin) is a narrow-spectrum β-lactam, semisynthetic derivative of penicillin. Methicillin acts by inhibiting the synthesis of bacterial cell wall. It inhibits cross-linkage between the linear peptidoglycan polymer chains by binding to 2  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady penicillin-binding proteins (PBPs). Methicillin is actually a penicillinase-resistant β- lactam antibiotic. Previously, it was used to treat infections caused by susceptible Gram-positive bacteria, in particular, penicillinase-producing organisms such as S. aureus, S. epidermidis, Streptococcus pyogenes, and Streptococcus pneumoniae. Figure 1: Classification of gram positive cocci 3  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady Introduction of methicillin into medical practice in the early 1960s quickly resulted in methicillin-resistant S. aureus (MRSA). Some MRSA are resistant to all but one or two antibiotics.Today, it is'nt as effective against these organisms due to resistance. MRSA is any strain of S. aureus that has developed resistance to β-lactam antibiotics, which include the Penicillins (Methicillin, Dicloxacillin, Nafcillin, Oxacillin, etc.) and the Cephalosporins. Strains unable to resist these antibiotics are classified as Methicillin-Sensitive S. aureus(MSSA). The evolution of such resistance make MRSA infection more difficult to treat with standard types of antibiotics and thus more dangerous. MRSA is treated by using vancomycin. Methicillin-resistant Staphylococcus aureus (MRSA) strains have emerged in the 1980s as a major clinical and epidemiological problem in hospitals. These strains are beginning to spread out of the hospitals and into communities. Virulence Factors of S. aureus The ability of S. aureus to cause disease has been attributed to an impressive spectrum of cell-wall-associated factors and Extracellular toxins as virulence factors. First: Capsule Second: Teichoic Acid Third: Staphylococcal Enzymes: Coagulase; Hyaluronidase ; Catalase; Fibrinolycin ; Lipase ; Nuclease and Penicillinase Fourth: Staphylococcal toxins: TSST-1; Exfoliative Toxins (Exfoliatin); Panton- Valentine Leukocidin(PVL(; Enterotoxins; and Cytotoxins. First : The Capsule protects bacteria by inhibiting the chemotaxis and phagocytosis. In addition, Capsule facilitates the adherence of bacteria to catheters and other synthetic materials. Second : Teichoic acid mediates the attachment of staphylococci to mucosal surfaces through its specific binding to fibronectin. Third: Staphylococcal Enzymes: Coagulase: converts the fibrinogen (in plasma) to insoluble fibrin, causing the staphylococci to clump or aggregate…. Detection of this enzyme is the primary test for identifying S aureus. What is the role of coagulase in the pathogenesis of S aureus ? the formation of fibrin layer (clot) around a staphylococcal abscess contributes in localizing the infection and protecting the organisms from phagocytosis. S. aureus possess two forms of coagulase: bound coagulase and free coagulase. 4  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady Fibrinolycin ( Staphylokinase): dissolves fibrin threads in the plasma clot, allowing S. aureus to free it 'self from clots, and spread. Hyaluronidase : hydrolyzes hyaluronic acid, (hyaluronic acid is the acidic muco- poly-saccharides present in the acellular matrix of connective tissues), thus, this enzyme facilitates the spreading of S. aureus through tissues… more than 90% of S. aureus strains produce this enzyme. Catalase: all staphylococci produce catalase which differentiates them from the streptococci. Catalase catalyzes the conversion of toxic hydrogen peroxide to water and oxygen…. Hydrogen peroxide can accumulate during bacterial metabolism or after phagocytosis. Lipase digests the lipids , allowing S. aureus to grow on the skin surface and in cutaneous oil glands. Nuclease (Nucleo-de-polymerase or Poly-nucleo-tidase) is an enzyme capable of cleaving the phosphodiester bonds between monomers of nucleic acids. Penicillinase ( β-Lactamase) breaks down penicillin, and allowing S. aureus to survive treatment with β-Lactam Antibiotics. Fourth: Staphylococcal Toxins Depending on the strain, S. aureus is capable of secreting several exotoxins. Many of these toxins are associated with specific diseases. Toxic Shock Syndrome Toxin( TSST- Exfoliative toxins(Exfoliatin Cytotoxins Enterotoxins Panton-Valentine Leukocidin(PVL( ============== Toxic Shock Syndrome Toxin (TSST-1 is a heat and proteolytic resistant. The ability of TSST-1 to penetrate mucosal barriers, even though the infection remains localized in the vagina or at the site of a wound, is responsible for the systemic effects of TSS. Death in patients with TSS is due to hypovolemic shock leading to multiorgan failure. Hypovolemic shock is an emergency condition in which severe blood and fluid loss make the heart unable to pump enough blood to the body. This type of shock can cause many organs to stop working. Exfoliative Toxins (Exfoliatin) causes a blistering of the skin known as staphylococcal scalded skin syndrome usually in infants. 5  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady 3 Cytotoxins (Heamolysins) Heamolysins cause lysis of red blood cells by destroying their cell membrane. In S. aureus heamolysins include α-hemolysin, β-hemolysin, and γ-Hemolysin. β -hemolysin also known as β -toxin, is the major cytotoxic agent released by S. aureus and the first identified member of the pore forming toxins. It's major function is development of pores in the cellular membrane which are able to cause the lysis of erythrocytes, leukocytes, and platelets. On Blood Agar plates, colonies of S. aureus are frequently surrounded by zones of clear beta-heamolysis( complete heamolysis). Enterotoxins: There are multiple (A–E, G–J, K–R and U, V) enterotoxins. Approximately %05 of S. aureus strains can produce one or more of them. The Enterotoxins are heat stable and resistant to the action of gut enzymes. Important causes of food poisoning, enterotoxins are produced when S. aureus grows in carbohydrate and protein foods and results in vomiting and diarrhea. once a food product has been contaminated with producing staphylococci and it 's enterotoxins, neither reheating the food nor the digestive process will be protective. Enterotoxin A is the most commonly associated with disease. Enterotoxin C and D are found in contaminated milk products, and Enterotoxin B causes staphylococcal pseudomembranous enterocolitis. 5 Panton-Valentine Leukocidin(PVL( This toxin of S. aureus can kill white blood cells of humans and rabbits. It has two components designated as S and F act synergistically on the white blood cell membrane and unlike the chromosomally encoded hemolysins above, PVL 6  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady is encoded on a mobile phage. This toxin is an important virulence factor in Community Acquired-MRSA infections. Antigenic Structure Staphylococci contain antigenic polysaccharides and proteins as well as other substances important in cell wall structure.  Peptidoglycan, a polysaccharide polymer containing linked subunits, provides the rigid exoskeleton of the cell wall.It is important in the pathogenesis of infection.  Teichoic acid.  Protein A is a cell wall component of S aureus strains and is a bacterial surface protein that has been characterized among a group of adhesins called Microbial Surface Components Recognizing Adhesive Matrix Molecules) MSCRAMMS). Bacterial attachment to host cells is mediated by MSCRAMMS, and these are important virulence factors. Protein A has become an important reagent in immunology and diagnostic laboratory technology; for example, protein A with attached IgG molecules directed against a specific bacterial antigen agglutinates bacteria that have that antigen (“coagglutination.(  Clumping Factor Another important member of MSCRAMM on the cell wall surface; clumping factor binds nonenzymatically to fibrinogen and platelets, yielding aggregation of the bacteria. ############################################ Pathogenesis of S. aureus The pathogenic capacity of S. aureus is the combined effect of extracellular factors and toxins together with the invasive properties. S. aureus expresses many potential virulence factors. (1) Surface proteins that promote colonization of host tissues. (2) Factors that probably inhibit phagocytosis (3) Toxins that damage host tissues and cause disease symptoms. Staphylococci can produce disease both through their ability to multiply and spread widely in tissues and through their production of many extracellular substances. Some of these substances are enzymes; others are considered to be toxins. Many of the toxins are under the genetic control of plasmids; some may be under both chromosomal and extrachromosomal control. Some of Staphylococci are members of the normal flora (microbiota) of the skin and mucous membranes of humans (Nasal carriage of S aureus occurs in 20–50% of humans); others cause suppuration, abscess formation, a variety of pyogenic infections, and even fatal septicemia. The pathogenic 7  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady staphylococci often hemolyze blood, coagulate plasma, and produce a variety of extracellular enzymes and toxins. The most common type of food poisoning is caused by a heat-stable staphylococcal enterotoxin. Staphylococci rapidly develop resistance to many antimicrobial agents, which consequently presents difficult therapeutic problems. S. aureus is able to cause many superficial pyogenic (pus forming) infections of the dermis and underlying tissues as well as serious systemic infections. Most Common Infections by S. aureus  Purulent Infections of the skin: Folliculitis, Furunculus, Carbunculus, Wound infections , Otitis Media, Mastitis.  Invasive Infections: pneumonia, Bacteraemia, Sepsis, Meningitis, Ostitis, Osteomyelitis, Endocarditis.  Toxin Medaited Infections: Gastroenteritis, Toxic Shock Syndrom(TSS), Staphylococcal Scalded Skin Syndrome(SSSS)  Abscesse formation. Treatment of S. aureus infections  Because of the frequency of drug-resistant strains, meaningful staphylococcal isolates should be tested for antimicrobial susceptibility to help in the choice of systemic drugs  Because pathogenic bacteria are commonly spread from one lesion (eg, a furuncle) to other areas of the skin by fingers and clothing, scrupulous local antisepsis is important to control recurrent furunculosis.  Serious multiple skin infections (acne, furunculosis) occur most often in adolescents. Similar skin infections occur in patients receiving prolonged courses of corticosteroids. Tetracyclines are used for long-term treatment.  Abscesses and other closed suppurating lesions are treated by drainage, which is essential, and antimicrobial therapy.  Acute hematogenous osteomyelitis responds well to antimicrobial drugs. In chronic and recurrent osteomyelitis, surgical drainage and removal of dead bone is accompanied by long-term administration of appropriate antibiotic.  Bacteremia, endocarditis, pneumonia, and other severe infections caused by S. aureus require prolonged intravenous therapy with a β-lactamase-resistant penicillin. Vancomycin is often reserved for use with nafcillin-resistant staphylococci.  Alternative agents for the treatment of MRSA bacteremia and endocarditis include newer antimicrobials such as daptomycin, linezolid, and Quinu-pristin, dalfo-pristin. 8  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady  Recently,a novel cephalosporin called ceftaroline, which has activity against MRSA and other gram-positive and some gram negative bacteria, has been approved for the treatment of skin and soft tissue infections. ########################################################### 2. Coagulase Negative Staphylococci (CoNS) Coagulase -negative staphylococci are normal human microbiota and sometimes cause infection, often associated with implanted devices, such as joint prostheses, shunts, and intravascular catheters, especially in very young, old, and immune- compromised patients. The three most frequently encountered species of clinical importance for this group are Staphylococcus epidermidis, S. saprophyticus, Staphylococcus lugdunensis, and Staphylococcus saprophyticus which cause approximately % infections caused by coagulase-negative staphylococci While, infections caused by S. lugdunensis, S. warneri, S. hominis, and other species are less common. Other species are important in veterinary medicine. It must always take into account that, already, CoNS are considered as nonpathogenic until their implication as nosocomial agents with emergence of multi- drug resistance which are responsible for high mortality rates worldwide. As the predominant nosocomial pathogen, CoNS presents a particular and continuing challenge for distinguishing between true infection and contamination.Recently, the studies support the view that CoNS should not be regarded as a skin contaminant but should be regarded as typical opportunists which represent one of the major nosocomial pathogens was having a substantial impact on human life and health. CoNS are particularly associated with the use of indwelling or implanted foreign bodies, which are indispensable in modern medicine. Colonization of different parts of the skin and mucous membranes of the host is the key source of endogenous infections by CoNS. However, they are transmitted mainly by medical and/or nursing procedures. Once inserted, foreign bodies can become colonized by CoNS and the success of the respective medical procedure is significantly impaired. S. epidermidis is a particular concern for patient with catheters or other surgical implants because it forms as a virulence factor that occurs most commonly on intravenous catheters and on medical prostheses. Common Infections with CoNS 1 Bacteremia Bacteremia is the presence of bacteria in the blood. Normally, Blood is a sterile environment, so the detection of bacteria in the blood (most commonly accomplished 9  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady by blood cultures) is always abnormal. Bacteria can enter the bloodstream as a severe complication of infections or during surgery (especially when involving mucous membranes such as the gastrointestinal tract), or due to catheters and other Foreign bodies entering the arteries or veins. In the hospital, indwelling catheters are a frequent cause of CoNS bacteremia and subsequent nosocomial infections, because they provide a means by which CoNS normally found on the skin can enter the bloodstream. With S. epidermidis, the area around the catheters may lack to overt signs of infection. 2 Bacterial Endocarditis Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves. Bacterial endocarditis occurs when the valves become infected with CoNS. Prosthetic valves are the most frequently affected by S. epidermidis, 3 Urinary Tract Infections (UTI) Urinary tract infections are commonly due to S. saprophyticus in young women. 4 Osteomyelitis Osteomyelitis is infection and inflammation of the bone and bone marrow. In immunocompromised patients, such as those on dialysis. Dialysis or haemodialysis is a process for removing wastes and excess water from the blood and is used primarily as an artificial replacement for lost kidney function in people with kidney failure. with indwelling vascular access catheters where the bacteria can sequester themselves in a biofilm, CoNS may also cause osteomyelitis of diabetic feet. S. lugdunensis can commonly cause osteomyelitis. 5 Other infections Infections with Devices that are exceptionally difficult to sterilize due to the inability of antibiotics to penetrate through the glycocalyx, such as:Prosthetic Joints; Cerebrospinal Fluid Shunts; Pacemakers; Ocular Implants; Cosmetic Implants These devices may all become infected with CoNS and most frequently need to be removed to achieve successful clearing of the infection. Biofilm Production and Pathogenesis of CoNS The pathogenesis of CoNS infections is mainly due to their abilities to produce biofilms on polymer surfaces and their multi-drug-resistant characteristic. A biofilm is any group of microorganisms in which cells stick to each other and often these cells adhere to a surface. These adherent cells are frequently embedded within a self- produced matrix of extracellular polymeric substance (EPS). Biofilms may form on 11  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady living or non-living surfaces and can be prevalent in natural, industrial and hospital settings The bacteria within the biofilms are protected from physical, chemical and biological stresses, including antimicrobial agents, antibodies and the antimicrobial products of phagocytic cells. The significance of the role of CoNS biofilm in a disease process was summarize as the followings:  Biofilm-grown bacteria are more resistant to adverse environmental conditions, such as lack of nutrients and competition from other bacterial species.  Biofilm cells are more resistant to the clearance of host immune system.  Cells in biofilms have dramatically reduced susceptibilities to antimicrobial agents.  Detachment of cells or biofilm aggregates may result in active bacterial dissemination and acute infections in neighboring tissues. Treatment of CoNS infections  Because of the frequency of drug-resistant strains, meaningful staphylococcal isolates should be tested for antimicrobial susceptibility to help in the choice of systemic drugs  S epidermidis infections are difficult to cure because they occur in prosthetic devices where the bacteria can sequester themselves in a biofilm. S epidermidis is more often resistant to antimicrobial drugs than is S aureus; approximately 75% of S epidermidis strains are nafcillin resistant.  Resistance to drugs of the erythromycin group tends to emerge so rapidly that these drugs should not be used singly for treatment of chronic infection. Nosocomial Infection Nosocomial Infection also known as a, Hospital-Acquired Infection (HAI), and it is sometimes instead called a health care–associated infection (HAI or HCAI). is an infection that is acquired in a hospital or other health care facility during hospitalization and which was not present or incubating at the time of admission, or, an infection which is acquired in the hospital and becomes evident after discharge from the hospital. Nosocomial Infection is spread to the susceptible patient in the clinical setting by various means. The Main Routes of transmission of Nosocomial infections included: First: Contact Transmission  Direct-Contact Transmission: a direct body surface-to-body surface contact such as patient-care activities that require direct personal contact 11  Medical Microbiology I st Second Stage/ 1 course / 2023-2024 Prof. Dr. Amal Talib Al-Sa'ady  Indirect-Contact Transmission :contact with a contaminated intermediate object, such as contaminated instruments, needles, or dressings, or contaminated gloves that are not changed between patients  Second: Droplet Transmission: droplets are generated from the source person mainly by coughing, sneezing, and talking, and during the performance of certain procedures, such as bronchoscopy.  Third: Airborne transmission, airborne droplet nuclei (droplets containing bacteria or viruses) or dust particles containing the infectious agent.  Fourth: Common Vehicle Transmission: by contaminated items, such as food, water, medications, devices, and equipment. ######################################################### Important Definitions  Pathogenesis of Bacteria is ability of an infectious bacteria to cause disease.  Adherence (Adhesion, Attachment): The process by which bacteria stick to the surfaces of host cells. it is a major initial step in the infection process.  Carrier: A person or animal with asymptomatic infection that can be transmitted to another susceptible person or animal.  Infection: Multiplication of an infectious agent within the body.  Invasion: The process where by bacteria, animal parasites, fungi, and viruses enter host cells or tissues and spread in the body  Non-pathogen: A microorganism that does not cause disease; may be part of the normal microbiota.  Opportunistic pathogen: A microorganism capable of causing disease only when the host’s resistance is impaired ( “immunocompromised”.  Pathogen: A microorganism capable of causing disease.  Toxigenicity: The ability of bacteria to produce a toxin that contributes to the development of disease.  Virulence: The quantitative ability of bacteria to cause disease. Virulence Involves adherence, persistence, invasion, and toxigenicity.  Virulent bacteria cause disease when introduced into the host in small numbers. Best wishes 12