Transcript Gram Positive Bacteria and Clinical Case Studies II
Gram Positive Bacteria and Clinical Case Studies II
Introduction
• Staphylococci and streptococci constitute the main group of medically important gram positive bacteria • There are also bacilli that are pathogenic such as Anthrax and the Clostridia group. These are also spore forming organisms • There are also some other gram positive rods such as Listeria, Lactobacillus,and Cornybacterium diptheria
Staphylococci
• Staphylococci stain dark purple with the gram stain.
• There are three medically important forms of staphylococci • Staphylococcus aureus – this causes many types of infections, food poisoning,and toxic shock • Staphylococcus epidermidis – this is a cause of infections in indwelling catheters • Staphylococcus saprophyticus that is an ongoing cause of cystitis in women
Staphylococci
• Found in the air and on surfaces • Very resistant to drying and dehydration • They produce Catalase which is one of their distinguishing characteristics* this is an enzyme vital to the survival of many aerobic bacteria • The most virulent form of staphylococcus, SA also secretes coagulase, that causes citrated plasma to clot. These are referred to as coagulase positive • There are other staph that do not have this ability and are labeled coagulase negative
Staphylococcus aureus
• Carried by 20-40 % of healthy individuals in their mouths or on their skin • It also found in their nasal mucosa • Carriers can serve as a source of infections as well as inanimate objects – these are called fomites
Staphylococci and virulence factors • Protein A – antiphagocytic effect • Exotoxins – Can cause cell lysis and destruction • Enterotoxins – Occurs when bacteria release toxin and cause vomiting • Toxic shock • Exfoliative –skin eruption and destruction
Infections
• Erythema • Cellulitis • Boils and carbuncles • Septicemia • Toxic shock • Food Poisoning • Indwelling catheters • Cystitis
Streptococcus
• Gram positive • Non motile • Ovoid to spherical in shape • Occur in pairs, short chains, or long chains
Biochemistry
• Many are facultative anaerobes • Ferment even in the presence of oxygen • Require nutrient rich environments • Identified by their growth patterns on blood agar
Streptococci
• Group A streptococcus – Streptococcus pyogenes – strep throat • Group B – streptococcus- genital infections Endocarditis – viridans group Otitis, and meningitis by
Streptococcus pneumonia
Blood agar – Alpha hemolysis
Hemolytic bacteria
• Cause a biochemical change in the hemoglobin of red blood cells – alpha hemolysis – green around the colonies • Cause gross hemolysis of the red blood cells in blood agar – beta hemolysis • Gamma – no hemolysis or change in the blood agar
Serologic
• Lancefields groupings This is based upon a carbohydrate, C, in the cell wall of the bacteria Bacteria are typed according to the variant
Streptococcus pyogenes
• Gram positive, non motile • Requires a low inoculum for infections • It does not survive well in the environment • Invades mucous membranes • Rapid progression of infection • Post infection sequelae can lead to glomerulonephritis
Transmission
• Present in nasopharyngeal region • Spread via aerosol droplets Like sneezing and coughing
Pathogenic features
• Fimbriae – for attachment- M protein • Exotoxins – cause rashes and other skin effects • Cytolytic toxins • Streptolysins – lyse white blood cells and red blood cells • Hyaluronidase – breaks down connective tissue to spread infection
Infections
• Strep throat • Rheumatic fever • Acute glomerulonephritis • Impetigo • Pharyngitis
Summary of differences between staphylococci and streptococci • Gram stain and morphology – Both Gram + – Staphylococci: bunched cocci – Streptococci: chained cocci (S. pneumoniae form
diplococcus)
• Enzyme tests – Staphylococci: catalase + – Streptococci: catalase - • Growth – Staph.: large colonies (non-fastidious), some
( hemolytic
– Strep.: small colonies (fastidious), many hemolytic a
or
b
)
Serological Techniques
• Lancefield classified the streptococci into 20 serotypes designated A through V.
• This is based upon an antigeninc substance called C • Streptococcus pyogenes belongs to Group A • These are also beta hemolytic
Bacitracin
• A filter paper disc impregnated with the antibiotic Bacitracin is placed on a blood agar plate containing bacteria • If there is a zone of inhibition around the disk it is indicative of a Group A streptococci
Serological technique
• Directigen test – A rapid nongrowth dependent immunological procedure • The bacteria are mixed with reagents that extract the Group A antigen and produce positive test results
Novobiocin Test
• Differentiates between staphylococci based upon senesitivity to the antibiotic Novobiocin.