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.

Molecular Methods - PCR

PCR Results