Skin microbiology Flashcards
Name the causes of skin disease
How does skin act as defence?
How does stimulation by superantigen e.g., staphylococcus sp. cause hypersensitivity reactions?
Staphylococcal superantigens (SAgs) bind directly to major histocompatibility complex (MHC) class II and T cell receptors to drive extensive T cell activation and cytokine release.
(The super antigens bypass normal process - leads to over activation)
Leading to massive cytokine release, causing a toxic shock due to subsequent over stimulation of numerous cytokine receptors. Or more local problems.
Give examples of microbial avoidance of defences
What is a commensal/resident flora?
can replicate on the skin and persist - normal to find and can be beneficial
What are nomad organisms?
colonise and reproduce on the skin for short times
What does transient mean?
Cannot replicate so stay for a short time
Give examples of some commensal bacteria found on the skin
Bacteria (e.g. staphylococcus sp., pseudomonas, proteus)
Yeast (malassezia sp.)
How do commensals prevent disease?
There are many commensals living on the skin that compete for habitable space, this prevents colonisation of the skin by pathogenic microbes or overgrowth of commensals.
Colonization by pathogenic microbes is often associated with low numbers of commensals or the skins physical integrity being compromised.
What is dysbiosis?
changes from balanced to unbalanced microflora
Give examples of resident bacteria found on canine skin
Staphylococcus epidermidis
Staphylococcus xylosus
Give examples of transient bacteria found on canine skin
Staphylococcus pseudintermedius
S. aureus
Proteus mirabilis
Pseudomonas spp.
Give an example of a resident yeast found on canine skin
Malassezia
Give examples of resident bacteria found on feline skin
S. aureus
S. pseudintermedius
Give examples of transient bacteria found on feline skin
Proteus mirabilis
Pseudomonas spp.
Staphylococcus spp.
What factors can influence infection?
Virulent pathogen
Dysbiosis
Penetration through damage of skin by commensals or pathogens
What factors impact the cutaneous environment?
What are staphylococcal bacteria?
Gram +ve
Facultatice anaerobes
Coccoid
Commensals
Opportunistic
Explain the pathogenecity of staphylococcus sp.
What virulence factors may staphylococcus spp. have?
Which staphylococcus species are of clinical importance to the skin?
S. hyicus - associated with epidermal infection of the pig (greasy pig disease)
S. pseudintermedius - commensal that can infect skin of cats and dogs
S. aureus - dermatological infections and one of the causes of mastitis
What are pseudomonas sp.?
Gram -ve
rods
aerobic
motile
pathogenic and non-pathogenic strains
common in environment
Describe the pathogenecity of pseudomonas spp.
Colonies are slimy due to polysaccharide mucus they secrete - this forms a biofilm in infection
Biofilms make them refractive to treatment
Burn infections, ear infections
What are biofilms?
Mix of bacteria and secreted polysaccharide
Aids adherence
Has role in infection - virulence factor
clinical relevance:
hard to clean out as sticky
harder for treatment
What is proteus mirabilis and when can it cause?
Gram -ve
rods
facultative anaerobes
motile
fishy smell
cause ear infection and UTIs
What is dermatophilosis
Actinomycete
Gram +ve
filamentous bacteria
forms zoospores
What is the pathogenesis of dermatophilius spp.?
Zoospores attracted to sites on skin
Germinate to produce hyphae, which penetrate living epidermis and spread in all directions
Penetrations -> acute inflammatory reaction
Invasion of epidermis ceases in 2-3 wks and lesions heal spontaneously
In chronic infections, the affected hair follicles and scabs are sites from which intermittent invasions of non-infected hair follicles and epidermis occur
What are the clinical signs associated with dermatophilosis?
Early papules
Followed by serous exudative dermatitis with scab formation
Serous exudate can cause matting of hairs
What are MRSP and MRSA
Methicillin resistant staphylococci (pseudointermedius and aureus) - rising concern
What do staphylococci spp contain that make them resistant to penicillin?
Beta-lactamase - breaks bond in beta-lactams
What does staphylococcus aureus contain that makes them more virulent and how does it work?
Coagulase
Reacts with prothrombin in the blood
S, aureus can coat its surface with fibrin and resist phagocytosis
What is the common genera of ringworm infections/hyphal fungal infections?
- microsporum sp
- trichopyton sp.
How do dermatophytes grow?
Describe yeast structure
eukaryotic
chitin cell walls
nutrition from excreting enzymes and reabsorbing material
unicellular
grow by budding
What are the common genera of yeast infections?
Malessezia pachydermatitis
Cryptococcus neoformans
Candida sp.
What are superficial, intermediate and deep mycoses?
Fungal infections
What is yeast found as a commensal?
external ear canals and superficial muco-cutaneous sites
What do yeast look like under a microscope?
Which species of dog have a higher malessezia level?
What is dermatophytosis?
Ringworm
What are dermatophytes?
Filamentous fungi
Asexual and sexual reproduction
grow exclusively in non-living tissue - skin, nails, hair
How do dermatophytes cause infection?
Infect keratinised areas e.g., hair (Invasion by digestion of keratin)
Hyphal growth as far as keratinisation of hair takes place
Hair breaks off due to weakening leading to partial alopecia
Inflammation leads to folliculitis
what is the most common cause of ringworm in cattle?
Trichophyton verrucosum
Risk is close animal contact
What is the most common cause of ringworm in horses?
Trichophyton equinum
Transmission by direct contact
What is most common cause of ringworm in cats and dogs?
Microsporum canis
can be asymptomatic in cats
What are some reservoirs for fungal pathogens?
Ringworm spores
Contaminated environment
Exposure to infected host
Fomites - e.g., shared tack/blankets
Why are cats that are asymptomatic with ringworm a risk?
Zoonotic
How are dermatophytes sampled and cultured?
Hairs selected are those that fluoresce, are broken, are along the edge of alopecic region
Dermatophyte test medium
What are common targets of antifungals
What are some diagnostic techniques in dermatology?
