Human Microbiota, Microbiome and Dysbiosis | Nursing Microbiology

Semester 1 | Microbiology | Chapter 7 | Unit 2 — Infection and Immunity

The human body is home to communities of microorganisms whose composition varies by body site, age, environment, diet, medications and health status. Resident microbes are not automatically harmful; many support barrier defences, metabolism and competition with potential pathogens. Nursing students need to interpret culture reports with an understanding of colonisation and of the limits of microbiome research.

1. Microbiota and microbiome are related but different

Microbiota refers to the microorganisms living in a defined habitat, such as the intestinal tract or skin. Microbiome is often used more broadly for the microorganisms and their genetic material, activities and surrounding environment. Different publications use the terms somewhat differently, so students should look at the precise definition in a study. A specimen result usually reports selected organisms—not a complete census of everything living on the sampled surface.

A person can carry a microorganism without having a disease caused by it. The term colonisation describes its presence and possible growth at a site without necessarily causing tissue damage or symptoms. Infection requires further clinical evidence. Knowing that a species can sometimes cause infection does not mean it must be eradicated whenever detected.

2. Body site determines normal community patterns

The skin presents diverse moisture, temperature, pH and lipid conditions, so microbial populations can differ between a dry forearm and a moist fold. The mouth is another rich habitat, supporting communities associated with teeth, saliva and mucosal surfaces. The large intestine contains a dense and diverse microbiota capable of interacting with dietary material and host metabolism.

Healthy body compartments are not interchangeable. Microbes that usually inhabit the large intestine may be clinically significant when detected in a normally sterile deep tissue sample. Conversely, detection on a non-sterile skin swab may be expected. Laboratory interpretation must consider how and from where the sample was collected and whether contamination occurred during sampling.

3. Intestinal communities and nutrition

Gut microorganisms ferment some dietary components that human digestive enzymes do not fully break down, producing metabolites such as short-chain fatty acids. Microbial communities can influence the processing of certain nutrients and signalling pathways in the intestinal environment. Yet changes in a population’s relative abundance do not always tell whether they caused a disease or merely changed alongside it.

Diet, illness, gastrointestinal transit and medications can alter observed microbial profiles. Research associations should not be converted into claims that a particular supplement, food or ‘microbiome reset’ will treat an individual condition. Nursing education should emphasise evidence quality, patient preferences and referral to licensed dietetic or medical care when appropriate.

4. Skin, mouth and respiratory mucosal communities

Skin communities live alongside physical barriers, antimicrobial substances and immune surveillance. Oral microbes can form dental biofilms, while normal upper respiratory microbiota are influenced by age and environmental exposure. Biofilm development is a common ecological strategy and is not equivalent to a diagnosis of infection. Infections can arise when microbes enter a different compartment or host defence is impaired.

Hand hygiene targets transmission of potentially harmful organisms rather than attempting to make living human skin permanently sterile. Clinical equipment and indwelling devices create different conditions from healthy mucosal surfaces. Sampling or device care must follow policy: a culture from a superficial swab cannot be treated as equivalent to an aseptically collected specimen from a normally sterile site.

5. Vaginal and urinary tract context

The vaginal microbiota in many reproductive-age people is often enriched in Lactobacillus species, although healthy community patterns vary among individuals and populations. Vaginal pH, hormones and life stages influence microbial ecology. A characteristic community pattern is not a universal definition of health, and test results must be interpreted according to symptoms and accepted diagnostic criteria.

The urinary tract is not simply a mirror of the intestinal microbiota. It is inappropriate to assume that any bacterium detected in urine necessarily indicates symptomatic infection. Patient age, pregnancy, procedures and specific clinical criteria can matter. Nurses should collect correctly labelled specimens and report symptoms accurately rather than treating a culture as a stand-alone diagnosis.

6. Colonisation resistance and host defence

Resident organisms can compete for nutrients and attachment sites, produce inhibitory molecules and interact with host mucosal immune mechanisms. These effects are sometimes called colonisation resistance. The microbiota works alongside barriers such as intact epithelium, mucus, stomach acidity and innate immune cells to reduce opportunities for some potential pathogens.

Protection is not absolute. A normally resident organism may itself cause opportunistic infection if it enters an unusual body site or if host defences become impaired. A sterile instrument may be contaminated by a patient’s own flora during an invasive procedure. A microbiology lesson should therefore avoid the simplistic division into ‘good bacteria’ and ‘bad bacteria’ without considering the situation.

7. Dysbiosis: a descriptive concept

Dysbiosis broadly refers to disruption or alteration of a microbial community compared with some reference state. It may involve diversity, relative abundance, metabolic activity or microbial interactions. However, there is no single universal reference community for all humans, and an observed change does not establish whether the change caused a disease.

Research uses sequencing and bioinformatics to detect broad patterns that routine clinical cultures often miss. Different sequencing methods can create different profiles. A commercial microbiome report therefore should not automatically be interpreted as proof of a disease or as an instruction to start antimicrobial medication.

8. Antibiotics and Clostridioides difficile

Antibiotics used to treat bacterial infection can also affect susceptible members of intestinal microbiota. In some circumstances, disruption reduces colonisation resistance and permits Clostridioides difficile to proliferate and cause illness. The CDC describes this relationship and recommends using antibiotics only as clinically appropriate, together with standard infection-prevention practices.

Not every episode of diarrhoea after antibiotics is caused by C. difficile, and positive tests must be interpreted according to current diagnostic criteria. A nurse should report new clinically significant diarrhoea, review possible medication exposure and follow local contact precautions and testing policies. Do not independently stop a prescribed medicine or label a patient with this infection.

9. Probiotics, prebiotics and evidence quality

Probiotics are live microorganisms administered in adequate amounts with an intended health benefit; prebiotics are substrates selectively used by host microorganisms in ways that may confer benefit. Effects are strain-, product-, indication- and population-specific. A result involving one strain cannot be extended automatically to every yogurt, capsule or fermented food product.

Some vulnerable patients, including particular severely immunocompromised or critically ill individuals, may face special risks from live microbial products. Nurses should not recommend them as universal alternatives to medical treatment. A claim about ‘boosting the microbiome’ needs evidence about the exact product, clinical outcome and patient population.

10. Nursing assessment and specimen interpretation

Assess symptoms, exposure and specimen quality before assigning meaning to a microbial name. The same species can be harmless in one site, contaminant in another and a cause of clinically significant infection in another. Laboratory culture conditions and diagnostic panels detect only certain organisms and may not represent an entire microbial community.

Explain to patients why an abnormal laboratory result does not necessarily mean they need antibiotics. Avoid promising that a commercial gut test or a diet change can prevent all infections. The central nursing principles remain safe collection, clear communication, appropriate hand hygiene, prescribed care and qualified clinical judgement.

Reference comparison table

Clinical scenarios: apply the principles

Scenario 1: Positive skin culture during routine review

The nurse distinguishes expected commensals from clinically meaningful infection using patient symptoms, wound findings, sampling quality and the treating team’s evaluation.

Scenario 2: Diarrhoea after recent antibiotics

The nurse documents frequency and clinical findings, alerts the team and follows local assessment and precautions; the symptoms do not prove C. difficile without evaluation.

Scenario 3: A patient asks for a probiotic recommendation

The nurse explains that evidence varies by strain and patient characteristics and refers product decisions to an appropriate professional, particularly with immunosuppression.

Clinical caution: These explanations cannot establish a patient diagnosis or justify independent antimicrobial or isolation decisions. Follow local standards, current professional guidance and qualified supervision.

Practice MCQs with answer explanations

1. Microbiota most precisely means?
   1. Microbes living in a specified habitat
   2. Only disease-causing microbes
   3. Only bacterial genomes
   4. Any antibiotic

   Answer A: Microbes living in a specified habitat. Microbiota is a community of microorganisms at a defined site.

2. Colonisation differs from infection because?
   1. It always requires antibiotics
   2. It only occurs on skin
   3. It may exist without causing symptoms or tissue injury
   4. It means no organisms are present

   Answer C: It may exist without causing symptoms or tissue injury. Clinical context determines whether colonisation has progressed to infection.

3. Which body site normally has a dense microbial community?
   1. Sterile central line lumen
   2. Large intestine
   3. Cerebrospinal fluid
   4. Healthy deep joint space

   Answer B: Large intestine. Intestinal contents normally contain extensive microbial communities.

4. Colonisation resistance can include?
   1. Only immune memory
   2. Only fever
   3. No microbial competition
   4. Competition for nutrients and attachment sites

   Answer D: Competition for nutrients and attachment sites. Resident communities can restrict establishment of other organisms.

5. Dysbiosis proves a particular disease?
   1. No; it is a descriptive change requiring context
   2. Yes in every case
   3. Only in children
   4. Always after vaccination

   Answer A: No; it is a descriptive change requiring context. A community difference is not proof of cause or a specific diagnosis.

6. Antibiotics sometimes raise C. difficile risk by?
   1. Producing viral capsids
   2. Making cells eukaryotic
   3. Disrupting protective gut communities
   4. Stopping all nutrient absorption

   Answer C: Disrupting protective gut communities. Antibiotic effects on intestinal communities can lower colonisation resistance.

7. A positive culture from non-sterile skin alone establishes infection?
   1. Yes
   2. No
   3. Only if Gram-positive
   4. Always after exercise

   Answer B: No. Consider signs, sampling technique and clinical evaluation.

8. A probiotic effect demonstrated for one strain applies to all probiotics?
   1. Yes
   2. Always for immunosuppressed patients
   3. Only with fever
   4. No

   Answer D: No. Evidence may be highly strain- and context-dependent.

9. What is a limitation of routine culture?
   1. It cannot reveal the entire microbiome
   2. It always identifies every virus
   3. It detects all bacterial genes
   4. It requires no patient identification

   Answer A: It cannot reveal the entire microbiome. Cultures detect organisms recoverable under the selected laboratory conditions.

10. Nursing priority before interpreting specimen significance?
    1. Choose antibiotics
    2. Ignore symptoms
    3. Check source, symptoms and collection quality
    4. Assume all flora is harmful

    Answer C: Check source, symptoms and collection quality. Specimen source and clinical context determine how results are evaluated.

Revision checklist

• Define microbiota, microbiome, colonisation and dysbiosis — describe the key concept without notes.
• Recognise broad patterns of microbial communities at skin, oral, gut and urogenital sites — describe the key concept without notes.
• Explain how normal microbiota contribute to host defence — describe the key concept without notes.
• Connect antibiotics and healthcare exposures to changes in microbial communities — describe the key concept without notes.
• Interpret microbiota claims without substituting them for patient-specific medical assessment — describe the key concept without notes.
• Reproduce the major distinctions in the comparison table and diagrams.
• Separate a preliminary microbial observation from a patient-specific clinical conclusion.

Sources and further reading

1. OpenStax — Prokaryote Habitats, Relationships, and Microbiomes
2. OpenStax — Physical Defenses
3. CDC — About C. diff Infection

Academic review: Original educational material awaiting Faizan’s review. Not independently clinically peer-reviewed.