Medical microbiology module 1

Colonization

Bacteria are on surface of body and do not cause harm to host

Media used for Staphylococcus

Blood Agar, Chocolate agar, Thioglycollate broth

Mannitol Salt Agar or MRSA ChromAgar as additional primary plating media

Infection

Bacteria break through barriers and multiple in deep tissue leading to an immune response or inflammation

Sir Alex Ogston

1800’s Scottish surgeon

Discovered greek staphyle (bunch of grapes) and kokkos (berry)

Fredrich Julius Rosenbach

1884 German physician

differentiated the bacteria by colony color: S.aureus (latin aurum gold) and S. albus

(latin for white)

Pathogenic Staphylococcus

staphylococcus aureus

staphylococcus epidermidis

staphylococcus saprophyticus

Staphylococcus lugdunensis

Staphylococcus pseudintermedius

Rarely pathogenic Staphylococcus

staphylococcus haemolyticus

staphylococcus intermedius

staphylococcus simulans

Staphylococcus capitis

Staphylococcus hyicus

Staphylococcus saccharolyticus

Micrococcus

Other name: M. luteus

Gram stain: Gram Positive: clusters, single, tetrads

Catalase: positive

Coagulase: negative

Test: modified oxidase is positive

Micrococcus

Located: Normal skin flora

Appearance: Yellow pigment on BA

Associated with: abscesses, septic arthritis, endocarditis, pneumonia & meningitis

Other tests: Microdase (modified oxidase) it detects cytochrome enzyme

Micrococcus is blue (+) Staphylococcus has no color (-)

Micrococcus gram stain

GPC in pairs, tetrads, and no fermenting glucose

Staphylococcus aureus

Located: Nares, axillae, vagina, pharynx, and skin surfaces

Gram stain: Gram Positive Cocci

Morphology: yellowish white, Beta hemolytic

Virulence factors: enterotoxins, toxic shock syndrome (toxin-1), Exfoliative toxin, cytolytic toxins, Enzymes, lipase, hyaluronidase, and Pro A

Catalase: positive

Plate growth: Blood agar, Chocolate agar, Mannitol Salt Agar, MRSA chromagar as additional culturing media

Abscess

Caused by: Staphylococcus Aureus

pocket of infection / tender mass that forms at the site of injury

-Area surrounding is red, painful and swollen

– Skin warm to the touch

– Usually filled with pus

Cellulitis

Caused by: Staphylococcus Aureus

Usually results from a scrape or cut in the skin which allows bacteria to enter

– Can occur anywhere in the body, but most often on the legs or arms.

– Symptoms include redness, swelling, and pain at the site of infection

Folliculitis

Caused by: Staphylococcus Aureus

inflammation of hair follicles due to an

infection, injury, or irritation

Impetigo

AKA School sores

Caused by: Staphylococcus Aureus

highly contagious skin

infection forming pustules and yellow, crusty sores

-Very common in children

– Most common skin infection in Northern Europe, U.S.A &

Canada

– > 3million cases per year in U.S.

– First sign is a patch of red, itchy skin

– Sores rapidly burst -> leaving yellow, brown crus

Boils / Furuncles

Caused by: Staphylococcus Aureus

Deep folliculitis

-painful swollen area on skin

– an accumulation of pus and dead tissue

Carbuncles

Caused by: Staphylococcus Aureus

Collection of furuncles

Called Skin and soft tissue infections SSTIs

Scalded Skin Syndrome

Caused by: Staphylococcus Aureus

Caused by toxin

New borns & young healthy children under 5

Adults: renal disease (high mortality)

Profuse peeling of epidermal layer (2- 4 days)

Toxic Shock Syndrome (TSS)

Caused by: Staphylococcus Aureus

Caused by toxin - Toxin 1 (super antigen → strong immune response)

production of cytokines → cytotoxic

Rare sudden high fever, a rash (like a sunburn on palms and soles), dehydration, shock

Risk factors: skin wounds, surgery, and tampons

Food poisoning

Caused by: Staphylococcus Aureus

Caused by toxin - enterotoxin A or D → GI problems with fever 2-8 hrs after ingestion

Resolved in 24-48 hours

Cramping, diarrhea, Headache

come from: mayo, meats, poultry, creamed filled pastries, dairy

DIAGNOSED BY LOCAL PUBLIC HEALTH LABS NOT HOSPITAL LABS

Exfoliative Toxin or epidermolytic toxin

Caused by: Staphylococcus Aureus

(strain) produced by phage group II

causes scalded skin syndrome aka Ritter’s disease

Bullous impetigo (fluid filled blisters)

Osteomyelitis

Caused by: Staphylococcus Aureus

an infection and inflammation of the bone (usually caused by bacteria). Can occur from an infection in bloodstream or post surgery

wound → Blood → bones

Pneumonia

Caused by: Staphylococcus Aureus

a serious lung infection & 2nd to viral / high mortality rate

Bacteremia

Caused by: Staphylococcus Aureus

common and serious condition in hospitalized patients and IV drug users

Septic Arthritis

Caused by: Staphylococcus Aureus

painful infection in a joint. It can come from bacteria that travel through the bloodstream from another part of the body

Necrotizing fasciitis

Caused by: Staphylococcus Aureus

“flesh eating disease”

Staphylococcus aureus DISEASES

Abscess

Cellulitis

Folliculitis

impetigo

Boils/ furuncles

carbuncles

Caused by toxin:

scaled skin syndrome aka ritter’s disease

Toxic shock syndrome

Food poisoning

Exfoliative Toxin

Virulence Factors of Staphylococcus Aureus: Cytolytic Toxin

affects RBC & Leukocytes (leukocidins)

4 hemolysis: α, β, delta, and gamma

α hemolysin lyzes platelets, macroph → severe tissue damage

β hemolysin → hot / cold lysin

delta hemolysin: found in more staph but less toxic

Gamma hemolysin - kills wbcs aka Panton valentine leukocidin

Severe cutaneous infections, necrotizing pneumonia

Virulence Factors of Staphylococcus Aureus: Enzymes

Enzymes: coagulase, protease, hyaluronidase, lipase

hyaluronidase- hyaluronic acic of conenctive tissue

Lipase- act on lipids on skin surface

facilitate spread of infection to adjoining tissue

PROTEIN A- found in cell wall of Staphylococcus Aureus

binds Fc portion of IgG to block phagocytosis

CNS

Coagulase Negative Staphylococcus

do not produce the enzyme coagulase

Staphylococcus epidermidis

Staphylococcus saprophyticus

Staphylococcus haemolytic

Staphylococcus hominis

Staphylococcus capitis

CNS is C H H E S!

staphylococcus epidermis

Located: Normal skin flora & mucous membranes

Diseases caused: UTI’s and endocarditis (prosthetic valve)

Virulence factors: Slime, Biofilm, contaminate catheters, shunts, and prosthetic devices

staphylococcus epidermis

Morphology: white, smooth, creamy

Hemolysis: non hemolytic

number one cause of blood culture contamination

Staphylococcus lugdunensis

Antibiotics: pan susceptible (vulnerable to most antibiotics)

Tests: PYR on sterile body site , Ornithine positive, Rapid staph (may be positive)

Morphology: Colonial morphology not definitive + may resemble s. aureus and s. epidermidis

Diseases caused: causes SSTI and endocarditis

Other tests: Rapid panel ID or MALDI

More virulent than other CNS

Staphylococcus saprophyticus

Located: Skin, Mucous membranes (genitourinary tract), rare on mucous membrane

Antibiotics: Novobiocin resistant

Diseases caused: UTIs in young sexually active women

Morphology: smooth, creamy + may produce a light yellow pigment

Catalase: positive

Coagulase: negative

Catalase test

Tests for enzyme catalase

2H2O2 breaks into 2H2O + O2

Differentiates catalase (+) Staphylococcus & micrococcus from catalase negative streptococcus

(+) bubbles present & catalase

(-) absent bubbles

Coagulase test

3 methods: Slide test, Tube test, and several commercial brand (Bacti staph)

Differentiates: Staphylococcus aureus (+) & Catalase Negative Streptococcus

(+) Clumping (fibrinogen → fibrin)

(-) clear background

Selective media

Mannitol Salt Agar (MSA): high salt content

Selective and differential: Staphylococcus aureus from Catalase Negative Streptococcus

(+) Staphylococcus aureus ferment mannitol and turns plate yellow

(-) Staphylococcus epidermidis remains pink

Antimicrobial susceptibility

Staphylococcus aureus is methicillin susceptible

Penicillinase resistance: Resistant to all beta lactam antibiotics (Methicillin, Penicillin, nafcillin, oxacillin, and amoxicillin)

Ex.) Methicillin Resistant Staphylococci Aureus is detected with the “Spectra” plate

(+) MRSA appears blue

MRSA & MRSE use vancomycin combined with rifampin or gentamicin

Expensive: Biomerieux Vitek 2, Microscan Walk-Away, BD Phoenix

MRSA

Methicillin Resistant Staphylococci Aureus

Located: colonizes patients nasal, axilla, and groin

Morphology: Weakly beta hemolytic or Non hemolytic

Antibiotics: Resistant to certain, commonly used antibiotics

Treated with vancomycin IV

Other tests: Diff / selective media (MRSA chromagar) 24 hrs + PCR test 1 hr

Cepheid GeneXpert (polymerase chain reaction)

HAI (Hospital acquired infection)

Infections that patients get while or soon after receiving health care

Causes: unclean hands, improper use or reuse of equipment between health care workers and patients

48 hours or more after patient is admitted

CAI (community acquired infection)

Infection contracted outside of a healthcare setting or an infection present on admission

1st case: Boston 1999

MRSA prevention

Regular handwashing

cover cuts and scrapes with a bandage until healed

Avoid contact with other people’s wounds / bandages

Avoid sharing personal items

Keep skin healthy and moisturized

Shower after sports

MSSA

Methicillin Susceptible Staphylococci Aureus

Susceptible to commonly used antibiotics

Treat with cefazolin IV

Resistance & Surveillance

mecA gene encodes for new penicillin binding protein

Passive: Isolate infected patients who are infected / colonized

Active: Screen patient before surgery to temporarily eradicate pathogen before it causes infection

Patient care impact

Patients can get hospital acquired MRSA → longer visit + more pain

Hospital acquired infection is 8th leading death in USA

Financial impact

MRSA cost $$$

long treatment + more equipment used (IV)

Drugs are expensive

Biofilm

Community of microbes attached to a solid surface

Ex.) plaque, biofilms on catheters

particularly resistant to antibiotics