2021.Bacteria Involved in Eye Infections (3).pdf

BACTERIA INVOLVED IN EYE INFECTIONS Department of Medical Microbiology 2 OBJECTIVES / OUTCOMES By the end of the lecture, the learner should be able to:  Describe the general characteristics, virulence mechanisms, laboratory diagnosis and diseases caused by the following bacteria:  Staphylococcus...

BACTERIA INVOLVED IN EYE INFECTIONS Department of Medical Microbiology 2 OBJECTIVES / OUTCOMES By the end of the lecture, the learner should be able to:  Describe the general characteristics, virulence mechanisms, laboratory diagnosis and diseases caused by the following bacteria:  Staphylococcus aureus  Streptococcus pyogenes  Streptococcus pneumonia  Pseudomonas aeruginosa  Haemophilus influenza  Moraxella catarrhalis  Neisseria gonorrhoeae  Chlamydia trachomatis OUTLINE OF THE LECTURE Gram positive & negative bacteria  Epidemiology  General characteristics  Virulence factors  Ocular infections  Laboratory diagnosis GRAM POSITIVE ORGANISMS : Staphylococcus aureus Streptococcus pyogenes Streptococcus pneumoniae 5 STAPHYLOCOCCI 6 Staphylococcus aureus EPIDEMIOLOGY  Family –Staphylococcaceae  Genus – Staphylococcus  Most commonly isolated organisms in the laboratory  Most clinically significant species of staphylococci  Primary reservoir is the human nares  Colonization also occurs in the axillae, vagina, pharynx, and other skin surfaces  Present in most environments frequented by humans  Transmission – direct contact with unwashed contaminated hands, and inanimate objects (fomites)  Carriers are source of infection to themselves and others 7 Staphylococcus aureus GENERAL CHARACTERISTICS  Gram positive cocci-clusters  Catalase producing, coagulase positive  Non-motile, non–spore-forming  Facultative anaerobic VIRULENCE FACTORS of S. aureus VIRULENCE FACTORS BIOLOGICAL ACTIVITY STRUCTURAL FACTORS Capsule Inhibits chemotaxis & phagocytosis Biofilm Facilitates adherence to foreign bodies; inhibits phagocytosis Peptidoglycan Stimulates production of endogenous pyrogens, osmotic stability Teichoic acid Binds to fibronectin Protein A Inhibits Ab mediated clearance by binding to receptors on Ab TOXINS Cytotoxins (α,β,γ) Toxic for erythrocytes, leukocytes, macrophages, platelets Exfoliative (Proteases) Split intra-cellular bridges in epidermis Enterotoxins(Superantigens Stimulate T cell production & release of cytokines Toxic shock toxin (TSST) Superantigen produces leakage & destruction of endothelial cells ENZYMES Coagulase Converts fibrinogen to fibrin Hyaluronidase Hydrolyses hyaluronic acids in connective tissue-promotes spread Fibrinolysin Dissolves fibrin clots Lipases Hydrolysis of lipids Nucleases Hydrolysis of DNA Staphylococcus aureus OCULAR INFECTIONS  Sties  Blepharitis  Keratitis  Conjunctivitis  Dacryocystitis  Periorbital cellulitis  Endogenous endophthalmitis 10 Staphylococcus aureus LABORATORY DIAGNOSIS  Specimen collection :  Depends on the clinical presentation  Swab, corneal scrapings and fluid.  Microscopy:  Gram positive cocci in clusters.  Culture:  blood agar: colonies produced after 18 to 24 hours of incubation appear cream-colored, white or rarely light gold  Selective media – Mannitol salt agar  Differential media- DNAse plate  Biochemical tests: catalase and coagulase positive. 11 IDENTIFICATION TESTS STREPTOCOCCI Streptococci EPIDEMIOLOGY:  Family – Streptococcaceae  Found in the nasopharynx and oropharynx of most healthy individuals  Colonization is highest in young children and their contacts  Most infections are endogenous  Spread by droplet  Disease results only when the host resistance is lowered by factors such as respiratory viral infections, pulmonary congestion/cardiac failure, malnutrition, immunodeficiency, smoking, stress, & alcoholism. Streptococci GENERAL CHARACTERISTICS  Gram positive cocci  Chains or pairs  Non motile  Some encapsulated  Facultative anaerobes  Some are fastidious  Catalase negative Streptococci Classification according to:  Antigenic structures: Lancefield Classification  Hemolytic patterns: Hemolysis on Blood Agar (BA)  Biochemical properties: Oxygen requirements  Anaerobic (Peptostreptococcus)  Aerobic or facultative anaerobic (Streptococcus) Haemolytic properties on blood agar -hemolysis Alpha  S. pneumoniae (Optochin susceptible)  Viridans strep groups (Optochin resistant)  Occasionally Group D and Enterococci Beta  Group A – Strep pyogenes  Group B – Strep agalactiae  Group C,F,G  Occasionally group D and Enterococci  Group K -hemolysis -hemolysis 1. Zone of "complete" (clear) haemolysis = ßhaemolysis 2. Zone of "incomplete" (green) hemolysis = α-hemolysis 3. No hemolysis = γ-hemolysis Gamma(non) Group D and Enterococci LANCEFIELD CLASSIFICATION Streptococci Lancefield classification Group A Group B Group C S. pyogenes S. agalactiae S. equisimitis Group D Enterococcus, Bovis group Other groups (E-U)  A to H, K to U  One or more species per group  Cell wall reactivity  Classification based on C- carbohydrate antigen of cell wall  Groupable streptococci  A, B and D (more frequent)  C, G and F (less frequent)  Non-groupable streptococci  Strep. pneumoniae (pneumonia)  Viridans streptococci  e.g. S. mutans - causes dental carries Streptococci OCULAR INFECTIONS  Streptococcal infections are an important cause of corneal ulcers, keratitis, endophthalmitis, conjunctivitis and dacryocystitis.  Streptococcal corneal ulcers and endophthalmitis were frequently associated with a poor visual outcome. Streptococcus pyogenes EPIDEMIOLOGY  Colonization: Skin, pharynx  Transmission - Respiratory droplets or skin contact  Group A streptococcal infections affect all ages peak incidence at 5-15 years of age  Accounts for 90% of cases of bacterial pharyngitis  Ocular: conjunctivitis and endophthalmitis Streptococcus pyogenes GENERAL CHARACTERISTICS  Gram positive cocci arranged in chains  Lancefield Group A  Capsulated  B-hemolytic Streptococcus pyogenes VIRULENCE FACTORS  Structural components  Immune evasion: M protein, which interferes with opsonization and lysis of the bacteria  Adhesins: Lipoteichoic acid, F & S protein (fibronectin)  Capsule: Hyaluronic acid which acts to camouflage the bacteria  Enzymes  Streptokinases  Deoxynucleases  C5a peptidase Facilitate the spread of streptococci through tissues  DNase B degrades DNA  Pyrogenic toxins that stimulate macrophages and helper T cells to release cytokines  Streptolysins (Exotoxins)  Streptolysin O lyses red blood cells, white blood cells, tissue cells and platelets  Streptolysin S lyses white blood cells. Not immunogenic Streptococcus pyogenes LAB DIAGNOSIS  Specimen collection: Swabs, scrapings and fluid from site of infection  Microscopy: Gram positive cocci in chains  Culture :  On sheep blood agar shows large zone haemolysis  Beta haemolytic colonies on blood agar  Small transparent colonies  Biochemical: Bacitracin disc is sensitive Streptococcus pneumoniae GENERAL CHARACTERISTICS:  Gram positive, slightly elongated diplococci (Lancet shaped)  Encapsulated  Non-motile & non-spore forming  Alpha hemolytic  Facultative anaerobes  Common cause of septicaemia meningitis; otitis media in children Streptococcus pneumoniae VIRULENCE FACTORS Structural components  Choline binding protein A - Enable the attachment to epithelial cells  Pili - Enable the attachment to epithelial cells  Polysaccharide capsule – Antiphagocytic and evasion of the immune system  Enzymes  Hyaluronidase - Facilitate the spread through tissue  IgA1 - Inactivates secretory IgA to facilitate pneumococcal colonization and subsequent invasion of mucosal surfaces  Toxins  Autolysin - hydrolyses the components of a biological cell in which it is produced. Mediates release of intracellular virulence factors (notably,pneumolysin)  Pneumolysin binds to cholesterol in the cell membrane mediate cell lyse through pore formation. Streptococcus pneumoniae LABORATORY DIAGNOSIS  Specimen collection: Specimens from site of infection  Microscopy: Gram positive diplococci, lancet shaped in pairs/chains  Culture: Blood agar is alpha haemolytic.  Optochin susceptible GRAM NEGATIVE ORGANISMS: Haemophilus influenzae Pseudomonas aeruginosa Moraxella catarrhalis Neisseria gonorrhoeae Haemophilus influenzae EPIDEMIOLOGY Strictly human pathogen Part of the human normal flora of the upper respiratory tract Transmitted from person to person by airborne route Introduction of the Hib vaccine has dramatically reduced invasive disease Colonization can persist for months, during which intercurrent upper respiratory infection may promote invasive disease and transmission Haemophilus influenzae EPIDEMIOLOGY Risk factors for invasive Hib infection include  Age