Staphylococcus aureus- Resistance, Epidemiology & Pathogenesis- Infections

Resistance

More resistant among the non-sporing bacteria

•          Remain viable for 3-6 months (even if isolated from dried pus after 2-3 months).
•          May withstand 60°C for 30 minutes (thermal death point: 62°C for 30 minutes)
•         Some require heating at 80°C for 1 hour to be killed-Heat resistant strains can grow at high temperature (45°C).
•          Most strains grow at 10-15% NaCl concentration (important in food spoilage).
•          Resist 1 % phenol for 15 mins.
•          Inhibited by Mercury peroxide 1% solution in 10 minutes.
•          Inhibited by bactericidal aniline dyes- Crystal violet (conc. 1 in 500,000) & Brilliant green (conc. 1 in 10,000,000)
•          Inhibited by fatty acids
•          Resistant to lysozymes
•          Initially sensitive to Penicillin, but with clinical use of penicillin, now resistant

• Penicillin resistance – 3 types

 1.Production of β lactamase (penicillinase)

•Inactivates penicillin by splitting β lactam ring

•Staphylococci produces 4 types of penicillinases (A-D) -Hospital strains usually produce type A penicillinase

•Penicillinase- inducible enzyme -Production usually controlled by plasmids, which are transmitted by transduction or conjugation. Same plasmid may carry genes for resistance to a range of other antibiotics and heavy metals

2. Alteration/Mutation in Penicillin binding protein (PBP) & changes in bacterial surface receptors

•Reduce binding of beta lactam antibiotics to cells

•Mutation normally chromosomal in nature

•Expressed more at 30°C than at 37°C

•Resistance extended to methicillin and cloxacillins (MRSA) – some even resistant to other antibiotics and heavy metals –main culprit in nosocomial/hospital infections

•Designated as EMRSA: “epidemic methicillin-resistant Staphylococcus aureus” resistant to erythromycins, tetracyclines, aminoglycosides and heavy metals

•Now, VRSA- vancomycin resistant strains also

 3.Development of tolerance to penicillin

• leads to inhibition of bacterium growth, and not death

 Epidemiology

•          Primary Pathogens of humans and animals- colonize skin, skin glands and mucus membranes
•          Human patients and carriers important source of infection than animals and inanimate objects (fomites)
•          10-30% healthy population – nasal carriers, 10% perineal carriers, 10% Hair carriers, 5-10% vaginal carriers (increases during menses –important in TSS)
•          Staphylococcal carrier state –Starts early in life
•         Colonisation of umbilical cord – neonates
•          Shedders: these carriers disseminate/spread/shed large numbers of Staphylococci for prolonged period.
•          Cocci shed by patients- contaminate fomites like Hand kerchiefs, bed linen, blankets – persists for days-weeks
•          Infected domestic animals such as cow also carry Staphylococci

•  Mode of infection

A. Exogenous infection – from patients or carriers

B. Endogenous infection - from colonized site of one’s own body part

  

Mode of transmission:

A. Direct contact

B. Indirect contact (fomites)

C. Dust

D. Droplet nuclei infection

 

Nosocomial infection-

·        - healthcare or hospital acquired infection

·        - Important since they are usually multidrug resistant

·         -Eg, MRSA- Methicillin resistant Staphylococcus Aureus, EMRSA., VRSA strains

·         -Common cause of post-operative infections and other hospital cross-infections

·         -These hospital strains are resistant to routinely used antibiotics in the hospitals

·         -Some of them cause epidemics of hospital cross-infections- epidemic strains

·        - Isolation of patients with staphylococcal lesions and strict aseptic measures including hand washing is to be followed to control Staphylococcal infections

·      -  Identification of carriers, eradication of virulent strain by deliberate spreading of strain of low virulence, antimicrobial prophylaxis by topical (surface) application of antiseptics etc found to be useful

Staphylococcus aureus - Pathogenicity

Staphylococcus aureus produces- Infections & Intoxications

Infections- Cocci gain access to damaged skin, mucosal or tissue site -Colonize by adhering to cells or extracellular matrix -Evade the host defense mechanisms and multiply - Liberating tissue damaging substances– Stimulate inflammation - Cause tissue damage

Intoxications-The diseases are caused by bacterial toxins – produced either in the host or preformed in vitro

1.    STAPHYLOCOCCAL INFECTIONS

§  Most common of bacterial infections- range from trivial to fatal

§  Characterised by localized pyogenic lesions (not spreading like streptococcal lesions)

§  Common Staphylococcal infections are:

a)  Skin and soft tissue infection

 Folliculitis – Furuncle (boil) – Abscess (breast abscess)- Carbuncle – Impetigo – Paronychia- less often, cellulitis- Wound infection

·         Folliculitis - It is inflammation of the hair follicles; A small red bump or pimple develops at infection sites of hair follicle

·         Sty: A sty is folliculitis affecting one or more hair follicles on the edge of the upper or lower eyelid.

·         Furuncle/boils: If infection extends from follicle to neighbour tissue, Furuncle. Thus it is deep seated infection, originating from folliculitis; Causes redness, swelling, severe pain- Commonly found on the neck, armpit and groin regions- common name- "boils"

·         Carbuncle: Carbuncle is an aggregation of infected furuncles; Carbuncles may form large abscesses; It is a large area of redness, swelling and pain, with pus.

·         Impetigo:  superficial skin infection, usually produces blisters or sores on the face, neck, hands, and diaper area - initially watery, then pus containing and finally honey coloured crust

        Paronychia: Nail inflammation 

     Cellulitis: Skin infection that causes redness, swelling, and pain in the infected area of the skin (organism enter through a crack or break in the skin)

b) Musculoskeletal infection

·         Osteomyelitis – inflammation of bone

·         Septic arthritis – knee, shoulder, hip

·         Pyomyositis – skeletal muscle infection

·         Bursitis (bursa- fluid filled sacs in the joints)

c) Respiratory

·         Tonsilitis

·         Pharyngitis

·         Sinusitis

·         Otitis – ear infection

·         Bronchopneumonia

·         Lung abscess

·         Empyema – pus in pleural cavity (lungs)

d) Central nervous system

·         Abscess

·         Meningitis

·         Intracranial thrombophlebitis – blood clot in cerebral vein

e) Endovascular

·         Bacteremia – bacteria in blood

·         Septicemia – blood stream infection

·         Pyemia – pus forming bacteria in blood stream (from an abscess)

·         Endocarditis – inflammation of heart valve

f) Urinary

·         Urinary tract infection – routine infections, also in association with local instrumentation, implants or diabetes