Skin Ecology and Indwelling Devices

Learning Outcomes

Examples of implants, infections, and sites.
Examples of common skin pathogens by cultural recovery.
Examples of gram-positive and gram-negative genera/species on different skin sites using modern molecular status.
Description of prosthetic hip infection and the techniques used to study.
Name a pathogen causing blockage of urinary catheters and factors involved in crystalline deposit formation/urinary stones.
Describe how pod formation in the bladder epithelium protects against host defenses.
Name a pathogen that exacerbates E. coli urinary tract infection.

Biofilms in Infections

Biofilms are microbial communities and a major infection problem.
The National Institutes of Health (NIH) stated in 2002 that 82% of all infections are biofilm-related.
Primary biofilm formation commonly occurs in the mouth (dental plaque, gum pathogens).
Breaking the skin barrier can lead to biofilm formation (e.g., intravenous catheters, artificial hip joints).

Common Implant Infections

Contact lenses: If not cleaned properly, can lead to biofilms with Staphylococcus epidermidis, causing keratitis.
Dentures: Candida albicans (yeast) is a cause of oral thrush.
Urinary catheters: Colonized by E. coli, Klebsiella, Enterococcus, and Proteus mirabilis.
Intravascular catheters: Problems with Staphylococci (epidermidis and Staph aureus) can cause septicemia and endocarditis.
Endotracheal tubes: Can lead to microbial problems, as seen during COVID-19.
Voice prostheses: Insertion of inanimate objects leads to biofilm formation (Streptococci).
Prosthetic valves and joint replacements: Bacteria can colonize surfaces, causing septicemia and device failure.

Oral Thrush

Candida albicans forms biofilms in the upper palate of denture wearers.
Microscopic view shows spores and long vegetative forms forming a mesh.
The mesh can trap other bacteria, causing a polymicrobial community.

Normal Skin Flora

Serves as a protective barrier.
Sebaceous glands secrete lubricant fluids (rich in urea, fatty acids, salts, lactic acid, and lipids).
Skin pH is normally acidic (pH 4-6, typically 5.5).
Hair follicles are habitats for aerobic and anaerobic bacteria, yeasts, and filamentous fungi.

Skin Flora Classification

Transients: Bacteria that are continuously inoculated but unable to multiply and eventually die.
Residents: Long-term inhabitants able to multiply on the skin.
Common skin infections: Staph aureus and Streptococcus.
Teenager spots are an example of a skin infection.

Historical Methods for Studying Skin Flora

Gold standard culture recovery involved swabbing and plating onto selective agars.
Primary bacteria recovered include Staph epidermidis, Streptococcus mitis, and Micrococcus species.
Aerobic and anaerobic bacteria such as Propionibacterium acnes (cause of acne) were also found.
Gram-negative bacteria were thought to be minor constituents.
Acinetobacter and John Solium were the only gram-negative bacteria regularly detected.
It was hypothesized that gram-positive bacteria outcompeted gram-negative bacteria.

Modern Molecular Sequencing and the Skin Microbiome

The Human Microbiome Project allowed for defining the skin microbiome using 16S ribosomal RNA sequencing.
The skin area is approximately two square meters, with different communities in different areas (head, chest, armpits, groin, feet).
Approximately 1000 bacterial species from 19 principal phyla are found.
Approximately 10^{12} bacteria are on the skin.

Composition of Skin Flora

Staphylococcus epidermidis and aureus make up about 5% of the flora.
Actinobacteria (e.g., Corynebacteria) make up over 50% of the flora.
Firmicutes (Bacilli, Staph, Streptococcus, Clostridium, Fusobacterium) account for 24%.
Proteobacteria account for about 17% (alpha, beta, gamma, delta, and epsilon subgroups).
Bacteroidetes (Prevotella and Flavobacteria) account for about 6%.
Pseudomonas aeruginosa (gamma proteome subgroup) can be a mutualistic bacterium by producing an antibiotic (MEU piracy) that inhibits Staph and Strep tRNA synthesis.

Major Sites and Flora

Sebaceous areas (hair and chest): Mainly Propionibacterium (Cutibacterium) and Staphylococcus species.
Moist areas (joints, groin, armpits): Mainly Corynebacteria and Staphylococci.
Dry areas (arms and legs): Mixture of beta proteobacteria and Flavobacteria.
Sebaceous areas have greater species richness due to more nutrients.

Breaching the Skin Barrier

Wounds or conditions like diabetes (causing ulcers, particularly around the feet) allow complex polymicrobial communities to establish.

Hip Infections

71,000 primary hip replacements in England and Wales, with 0.6% suffering deep infection.
231,000 hip replacements in the USA, with 0.9% suffering deep infection.
Infections cause extreme pain and prolonged immobilization.
Treatment involves antibiotics, but infections may lead to persistent biofilms.
Surgical revision involves removing the original prosthetic joints and debriding infected tissue.
One-stage revision involves immediately putting in a new joint (30% of cases).
Two-stage revision involves taking the joints out, starting antibiotic therapy, and then putting the new joints in (more effective but costs 70% more).
Even with surgical revisions, there is a 10% re-infection rate due to established biofilms.

Detection of Biofilms Using PET Scans

18 fluorine fluoro-deoxy glucose accumulates in activated inflammatory cells at sites of infection.
PET scans can visualize biofilm accumulation on implants.

Visualizing Biofilms on Hip Joints

After removal of hip joint components, slimy material (biofilm) can be observed.
Microscopic analysis reveals bacteria (stained green) and EPS.

Problems with Indwelling Devices

Catheters breaching the skin can lead to infections, particularly if sterility is compromised.
Central line catheters are invasive procedures that can carry blood-borne pathogens to the heart.
Biofilms can form at the site of intravenous catheter insertions, leading to systemic infections.
An example showed Cryptococcus neoformans spreading from a catheter site to the lungs and causing meningitis.

Catheters and Urinary Tract Infections (UTIs)

Catheters are a major cause of UTIs.
Inserted through the urethra into the bladder for continuous urine drainage.
Breaching the body's defenses and difficult to keep sterile.

Biofilm Formation

Microorganisms attach to a surface and form colonies, leading to mature biofilms.
Parts of the biofilm can detach and move to new locations.
The biofilm is a safe haven, protected from antibiotics, disinfectants, and the immune system.
Translocation involves bacteria moving against the flow in static zones away from the flow, called laminar flows.

Catheters as Portals for Infection

Foley catheters have a balloon on the end that is inflated to form a seal at the base of the bladder.
The tip of the catheter has holes for urine drainage, but it can also serve as a portal for infection.

Catheter Blockage

In some cases, particularly with Proteus, catheters can become blocked.
Crystalline deposits form inside the lumen, blocking the flow of urine.
Crystalline deposits are made up of struvite (magnesium ammonium phosphate) and apatite (hydroxylapatite in calcium phosphate).
Crystals grow because bacteria generate crystal formation.

Proteus mirabilis and Urease

Proteus mirabilis is the pathogen responsible for catheter blockage.
It produces urease, which hydrolyzes urea to form ammonium and carbonate ions, increasing the urinary pH.
High pH causes precipitation of magnesium and calcium phosphate crystals.
Urease and struvite break down the urethral surface.
Proteus mirabilis can cause recurrent urinary catheter infections.

Consequences of Catheter Infections

Ascending reflux can lead to kidney and bloodstream infections (pyelonephritis, septicemia, toxic shock).
Aggregates can form stones in the bladder, resistant to antibiotics.
Even after removing an encrusted catheter, the biofilm is already established in the bladder, and stones are still forming.

Strategies to Lower Urine pH

Citrus drinks, cranberry juice, and acidic fruit juices can increase fluid uptake to wash out the urinary tract.
Surgeons may need to surgically remove bladder stones if antibiotics fail.

Antimicrobial Catheters

Silver-impregnated catheters have been explored to reduce biofilm formation.
However, studies have shown that silver does not prevent biofilm formation or kill established biofilms.

Mouse Bladder Infection with Proteus

Stones can penetrate down into the epithelial layers of the bladder.

Pathogenic Factors of E. coli and Proteus mirabilis

E. coli causes relatively uncomplicated UTIs, while Proteus mirabilis causes more complicated UTIs.
Both produce virulence factors such as fimbriae and pili to attach to epithelial surfaces.
Both make toxins that attack host tissues and evade immune defenses.

Biofilms in the UTI Process

Latex catheters have rough surfaces with niches for bacteria to establish.
Staphylococcus aureus forms biofilms on latex catheters.
E. coli produces alginate, resulting in a lot of extra polymeric substance in the biofilm.
Proteus forms biofilms in urine with a basal layer and long fronds of microcolonies.
Proteus exhibits swarming behavior due to its flagella.

Immune System and Bladder Infection

Leukocytes attack in E. coli infections in the mouse bladder.
Stones act as nuclear bunkers for E. coli, providing a physical barrier against immune attack.
E. coli produces Aji 43, characteristic of biofilm formation in the bladder.

Co-infection with Gardnerella

Gardnerella, found in the vagina, can cause problems in the urinary tract.
It can attack the epithelial surface of the GI tract, exposing the underneath to an E. coli UTI.
E. coli does better in polymicrobial infections.

Summary

Skin flora is complex, and breaching the barrier leads to disease.
Indwelling devices breach the body's defenses, allowing what gets in to form biofilms.
Current materials have little advantage in suppressing biofilm formation.
Biofilms provide defense against immunity and antibiotics.
Biofilms are linked to inflammatory responses and cystitis.
Polymicrobial infections increase the virulence of some pathogens.