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Gram Positive BacteriaandClinical 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 dropletsLike 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 ( or )
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.
Molecular Methods - PCR
PCR Results