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Microbiology Project Topics

Bacterial Colonization of Staphylococcus Aureus on Human Skin

Bacterial Colonization of Staphylococcus Aureus on Human Skin

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Bacterial Colonization of Staphylococcus Aureus on Human Skin

CHAPTER ONE

OBJECTIVES

The main objective of this study was to investigate the molecular mechanisms underlying the determinants involved in S. aureus colonization and/or infection of its host. We aimed to provide increased knowledge regarding the role of the S. aureus CWA proteins SdrD and SasG in bacterial adhesion to host cells and/or evasion of the host immune defence mechanisms.

Ourย specificย researchย questionsย were:

  1. Does S. aureus SdrD interact with specific host ligand(s) in the epithelium? What could be the role of such ligand interaction?
  2. How conserved is the sdrD gene within S. aureus isolates from healthy individuals? Could variation within the sdrD gene influence SdrD function?
  3. Does S. aureus SasG contribute to host cells adhesion to human keratinocytes? Under what conditions is SasG expressed? Does S. aureus SasG have immune evasion properties?

CHAPTER TWO

AUREUSย COLONIZATION

Significanceย ofย Colonization

Humans are constantly exposed to S. aureus in their environment and our body provides a range of ecological niches for the S. aureus and other microbes to thrive56. However, not every exposure to S.ย aureus will lead to successful colonization. S.ย aureusย colonization of its host involves a complex interplay of factors from the bacterium and its host57. Longitudinal studies have shown that 20-30% of the healthy adult population is persistently colonized by the S.ย aureus in their anterior nares1,2,58,59. S.ย aureus colonization requires that the bacterium isย ableย toย adhereย toย theย receptorsย presentย atย theย ecologicalย niches.ย Furthermore,ย itย mustย alsoย be able to thrive and not be eradicated by the host defence mechanisms or resident microbiota60,61.

  1. aureus colonizationย isย anย importantย andย essentialย riskย factorย forย subsequentย developmentย of
  2. aureus infection and hospital acquired infections1,4,62,63. Danbolt established the first correlation between nasal carriage and furunculosis skin infection in 1932 (reviewed in23). In addition, correlation between S. aureus nasal carriage and infections such as continuous peritoneal dialysis (CPD) related infections64, HIV65, post-operative infections66,67and foot ulcer68have been observed. Studies have shown that the infecting strains are S.ย aureus strain, which had colonized its carriersโ€™ nares62,69,70. In addition, patients colonized with MRSA before hospital admission, have a higher risk of developing MRSA infections71โ€“73ย and serve as depots for transmission to other patients74.

Sites ofย ย aureus Colonization

Theย primaryย ecologicalย nicheย forย S.ย aureusย colonizationย inย humanย isย theย nose63,75. However,ย aureushave been reported to colonize other sites within the human body including skin75, perineum76, vagina77, axillae78, pharynx75,77, gastrointestinal tract75,79,80, urinary tract and throat81,82. Exclusive S. aureus throat, intestinal and pharynx colonization without nasal carriage have been reported79,81โ€“83. In addition, some studies show higher incidences of S. aureusย prevalenceย inย theย throatย andย pharynxย comparedย toย theย nasalย carriage81,84.ย Theย abilityย ofย S.ย aureusย toย surviveย inย differentย ecologicalย nichesย ofย theย humanย bodyย showsย itsย versatilityย and diversity in colonizing its host.

Nasal Colonizationย byย ย aureus

aureus nasal carriage influences the bacterial colonization of other parts of the human body85. This implies that S. aureus nasal carriage most likely serve as a repository for the dispersalofS.ย aureusย intoย environmentย orย colonizationย ofย otherย bodyย parts86.ย Habitsย such asย nose picking could be an avenue for transfer ofย S.ย aureus carried in the nose to other areas of the human body87. In addition, patients and healthcare workers nasally colonized by S. aureus can also spread the bacterium to non-colonized persons in hospital settings74,88.

Based on the risk posed by S. aureus nasal carriage, calls to develop effective nasal decolonizationย strategies haveย increased89,90.ย Decolonizationย of S.ย aureusย inย theย anteriorย nares followingย coursesย ofย intranasalย applicationย ofย theย antibioticsย mupirocinย has beenย reported91,92. In addition, nasalย decolonization treatment also eliminated S.ย aureus from the hands of health workers93. Eradication of S. aureus in the anterior nares in the patients reduced theย occurrence of S. aureus infections94โ€“96. Application of mupirocin has also been used to eradicate MRSA carriage97. These observations further strengthen the notion that the nasal environment provides a very viable environment for the colonization and subsequent propagation of S.ย aureus. However, despite the success of mupirocin in eradicating S.ย aureus nasal colonization, S. aureus has developed resistance to the antibiotic98.

In human nose, the main ecological niche of S. aureus is the moist squamous epithelium of the anterior nares of healthy adults in a general population2,63,99. This has been further supported by in vitro cell studies, which showed an increased adherence of S. aureus to desquamatedย epithelialย cellsย isolatedย fromย theย anteriorย nares100,101.ย However,ย S. aureusย also colonize other regions nose as well, from mid region nares to the deeper regions of the nose102. Interestingly, Kaspar et al., observed within the sampling population of their study that the posterior region of the nose was consistently colonized compared to the anterior nares103. The surface of the anterior nares is lined with a skin-like epithelium while theย middle and posterior region of the nose is lined with pseudostratified columnar ciliated epithelium102,104,105. The role of these different surface cellular constituents on nasal microbiota has been suggested102. However, in another study where the human nasal microbiome evaluated, they concluded that the epithelium constituent does not affect the nasal microbial diversity103, butย a large proportion of the participantsย in the study had chronic nasal inflammation.

 

CHAPTER THREE

METHODOLOGY

A detailed description of the experimental procedures performed in this thesis is listed and explained in the articles and manuscript herein. An overview of some of the experimental procedures performed is explained below.

Hostย Protein-Pathogenย Proteinย Interaction

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Protein-protein interaction detection assays have been employed to determine the interaction between host proteins and S. aureus virulence factors. Elucidating these interactions is particularlyย helpfulย inย decipheringย someย ofย theย molecularย mechanismย engagedย byย S.ย aureusย in colonizing humans. It has been suggested that this might form the basis for the development of some anti-colonization strategies in future297. In Paper I, a yeast two-hybrid (Y2H) assay298ย was ordered from Hybrigenics to find the potential host partners forย S.ย aureus SdrD protein. Basically, Y2H assays are performed in yeast strains containing two proteins. One of the proteins referred to as โ€œbaitโ€ is fused with a DNA binding domain (DBD) while the other protein referred to as โ€œpreyโ€ is fused withย an Activation domainย (AD). Physical interaction of the two proteins brings these two domains in close proximity and thus forming a functional transcription factor. The transcription factor then activates the reporter gene, which can be assayed by growing the yeast strain on a selective medium or observing for colorimetric changes. In Paper I, the SdrD A region fused with GAL4DBD was the bait protein and this was used to screen the human reconstituted skin libraries fused with GAL4AD (the โ€œpreyโ€). We selected Y2H assay because it allowed for screening of large proteins libraries for SdrD putative host partners. It is important to note that Y2H assay has some limitations, one of which is the high false positive rates of potential candidates299. However, in Paper I, weย have employed complementary methods to validate the putative partners as indicated by the Y2H.

CHAPTER FOUR

GENERALย DISCUSSION

ย 

Adequate understanding of the mechanisms of S. aureus colonization is vital in developing alternatives strategies to combat its ability to cause infections andย diseases. Host colonization by S. aureus involves a complex interaction of diverse factors, which are the pathogen, host and the environment23,316. In this thesis, mechanisms of some of the bacterial determinants involved in S.ย aureus colonization and infection were investigated. Particularly, we focused on the role of the S.ย aureus CWA proteins SdrD and SasG in adhesion and immune evasion processes.

CHAPTER FIVE

CONCLUSION

ย 

Though our understanding of S. aureus interaction with humans has improved greatly since the discovery of the bacteria, there are still a lot of questions regarding the molecular determinantsย involved.ย Despiteย this,ย S.ย aureusย remainsย anย efficientย colonizerย andย anย important human pathogen. Host colonization and/or infection by S. aureus is both complex and multifactorial. This study has characterized the role of S. aureus cell wall anchored proteinsย in ensuring its adhesion and/or immune evasion.

 

We showed that CWA proteins SdrD and SasG mediate bacterial adhesion to host cells (Paper I, II and III). Furthermore, we identified Dsg1 as the host ligand of SdrD and show thatย thisย interactionย promotesย bacterialย adhesionย toย hostย cellsย (Paperย I).ย Inย addition,ย weย found that genetic variations within the sdrDย gene is concentrated within its SdrD A domain and R domain (Paper II). Furthermore, we revealed that expression of S. aureus SasG is upregulated in human blood and in the presence of serum components (Paper III). However, we found that SasG does not promote bacterial survival in human blood ex vivo (Paper III).

ย 

Our findings are consistent with the multifactorial nature of S. aureus-host interaction. We have provided increased knowledge about the molecular mechanism S. aureus uses in its interaction with the host. Additional studies into the subsequent events following this interaction is needed in order to further clarify the mechanism used by S.ย aureus to colonize and invade host cells.

 

In the future, our findings (Paper I, II and III) should help in providing molecular mechanistic knowledge required in developing alternative therapeutics to combat S. aureus colonization and/or infection.

 

REFERENCES

ย 

  • Kluytmans, J. et al. Nasal carriage of Staphylococcus aureus: epidemiology, underlyingmechanisms,ย andย associatedย ย Clinicalย microbiologyย reviewsย 10,ย 505โ€“ 20 (1997).
  • vanBelkum,ย ย etย al.ย Reclassificationย ofย Staphylococcusย aureusย Nasal Carriage Types. Theย Journalย ofย Infectiousย Diseasesย 199,ย 1820โ€“1826 (2009).
  • Wertheim, F.ย etย al.ย Theย roleย ofย nasalย carriageย inย Staphylococcusย aureusย infections. Lancetย Infectiousย Diseasesย 5,ย 751โ€“762ย (2005).
  • Wertheim,ย F.ย etย al.ย Riskย andย outcomeย ofย nosocomialย Staphylococcusย aureus bacteraemiaย inย nasalย carriersย versusย non-carriers.ย Lancetย 364,ย 703โ€“705ย (2004).
  • Foster,ย J.ย etย al.ย Adhesion,ย invasionย andย evasion:ย theย manyย functionsย ofย theย surface proteins of Staphylococcus aureus. Natureย reviews. Microbiologyย 12, 49โ€“62 (2014).
  • Geoghegan, J. A. & Foster, T. J. Cell wall-anchored surface proteins of Staphylococcusaureus:ย Manyย proteins,ย multipleย ย inย Currentย Topicsย in Microbiology and Immunology 409, 95โ€“120 (2017).
  • Pendleton,ย N.ย etย al.ย Clinicalย relevanceย ofย theย ESKAPEย pathogens.ย Expertย Reviewย of Anti-infective Therapy 11, 297โ€“308 (2013).
  • Rice,ย B.ย Federalย Fundingย forย theย Studyย ofย Antimicrobialย Resistanceย inย Nosocomial Pathogens:ย Noย ESKAPE.ย Theย Journalย ofย Infectiousย Diseasesย 197,ย 1079โ€“1081ย (2008).
  • Tong, S. Y. C. et al. Staphylococcus aureus infections: epidemiology, pathophysiology,clinicalย manifestations,ย andย ย Clinicalย microbiology reviews 28, 603โ€“61 (2015).
  • Chambers,ย F.ย &ย DeLeo,ย F.ย R.ย Wavesย ofย resistance:ย Staphylococcusย aureusย inย the antibiotic era. Nature Reviews Microbiology 7, 629โ€“641 (2009).
  • Ogston,ย Reportย uponย micro-organismsย inย surgicalย diseases.ย Britishย Medicalย Journalย  1, 369โ€“377 (1881).
  • Ogston,ย Micrococcusย Poisoning.ย Journalย ofย Anatomyย andย Physiologyย 17,ย 24โ€“58 (1882).\
  • Rosenbach,ย J.ย Mikro-organismenย beiย denย Wund-Infections-Krankheitenย des Menschen. (J.F. Bergmann, 1884).
  • Marshall,ย H.ย &ย Wilmoth,ย G.ย J.ย Pigmentsย ofย Staphylococcusย aureus,ย aย seriesย of triterpenoid carotenoids. Journal of Bacteriology 147, 900โ€“913 (1981).
  • Foster,ย Staphylococcus.ย inย Medicalย Microbiology.(4ย thย ed.)ย Galvestonย (TX):ย (ed. Baron S) 1โ€“11 (University of Texas Medical Branch at Galveston, 1996).

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