What you'll learn
- What hospital-acquired infections are, and why hospitals can make transmission more likely.
- How the “chain of infection” helps you choose suitable control measures.
- Why antibiotic resistance, including MRSA, is a major hospital problem.
- How to interpret infection-rate data and evaluate control strategies.
Starting point: what counts as an infection?
A hospital is designed to treat illness, but it also contains many vulnerable people, many microorganisms, and many opportunities for transmission. To understand infection control, start with the basic vocabulary.
Key terms
- A pathogen is a microorganism that can cause disease, such as a bacterium, virus, fungus or protoctist.
- An infection occurs when a pathogen enters the body, multiplies, and causes damage or triggers symptoms.
- Colonisation means microorganisms are present and multiplying on or in the body, but are not currently causing disease.
- A hospital-acquired infection (HAI) is an infection acquired in a hospital or healthcare setting, not present or incubating when the patient was admitted.
- The incubation period is the time between infection and the appearance of symptoms.
HAIs are often identified if symptoms appear 48 hours or more after admission, but that is a useful guideline rather than a perfect rule. The key idea is whether the infection was linked to healthcare exposure.
Deciding if an infection is hospital-acquired
A patient is admitted for planned knee surgery with no signs of infection. Three days later, the surgical wound becomes inflamed and a bacterial culture identifies Staphylococcus aureus.
- Compare the timing with admission: symptoms appeared about 72 hours after arrival, so the infection was not obvious at admission.
- Consider the site: the infection is in the surgical wound, so a hospital procedure may have provided a route of entry.
- Link the evidence: because the timing and wound site both point towards healthcare exposure, this is likely to be classified as a hospital-acquired infection.
Why hospitals increase infection risk
Hospitals contain many people with reduced defences. A person may be immunocompromised, meaning their immune system is less able to respond effectively. This can happen because of age, chemotherapy, malnutrition, HIV infection, burns, major surgery or other illness.
Hospitals also use procedures that bypass natural physical barriers. For example, a catheter can provide a route into the urinary tract, an intravenous line can provide a route into the blood, and surgery breaks the skin barrier.
Why HAIs happen
A hospital-acquired infection usually needs three things: a pathogen, a route of transmission, and a susceptible host.
The normal microbiota — the harmless or beneficial microorganisms normally living on and in the body — also matters. Broad-spectrum antibiotics can kill many normal gut bacteria, reducing competition and allowing pathogens such as Clostridioides difficile to multiply.
The chain of infection
A useful model is the chain of infection. Each “link” is a stage needed for transmission. Infection control works by breaking one or more links in the chain.
The six links are:
- Infectious agent — the pathogen causing disease.
- Reservoir — where the pathogen lives, such as a patient, healthcare worker, sink, bed rail or medical equipment.
- Portal of exit — how the pathogen leaves the reservoir, such as coughing, blood, faeces or wound fluid.
- Mode of transmission — how it spreads, such as direct contact, droplets, contaminated surfaces or contaminated equipment.
- Portal of entry — how it enters another person, such as through the mouth, respiratory tract, wound or catheter.
- Susceptible host — a person whose defences are not strong enough to prevent infection.
Breaking transmission from a catheter
A ward has several urinary tract infections linked to urinary catheters.
- Identify the likely portal of entry: the catheter passes into the urinary tract, so it can allow bacteria to bypass normal external barriers.
- Identify the likely transmission route: bacteria may be transferred from hands, skin or equipment during insertion or catheter care.
- Choose measures that target the weak points: use aseptic insertion, sterile equipment, hand hygiene, gloves, regular catheter review, and remove the catheter as soon as it is no longer needed.
Common hospital-acquired infections
MRSA
MRSA stands for methicillin-resistant Staphylococcus aureus. S. aureus can live harmlessly on the skin or in the nose, but it can cause wound infections, pneumonia or bloodstream infections if it enters deeper tissues.
MRSA is important because it is resistant to several beta-lactam antibiotics, so treatment options are more limited. It spreads mainly by contact, especially via hands or contaminated surfaces.
Clostridioides difficile
Clostridioides difficile is often still called Clostridium difficile in older resources. It can cause severe diarrhoea, especially after antibiotics disrupt the normal gut microbiota.
It forms endospores, which are tough dormant structures that help the bacterium survive harsh conditions. These spores can persist on surfaces and are not reliably removed by alcohol hand gel alone.
Alcohol gel and spores
Alcohol hand gel is useful for many pathogens, but C. difficile spores require thorough handwashing with soap and water, plus appropriate sporicidal cleaning of surfaces.
Other examples
Other HAIs include surgical-site infections, catheter-associated urinary tract infections, ventilator-associated pneumonia, norovirus outbreaks and bloodstream infections linked to intravenous lines.
Antibiotic resistance in hospitals
Antibiotic resistance evolves by natural selection. In a bacterial population, some cells may already carry a resistance allele due to mutation or may acquire a resistance gene on a plasmid, which is a small circular DNA molecule found in bacteria.
When an antibiotic is used, susceptible bacteria are killed or stop reproducing. Resistant bacteria survive, reproduce, and may pass resistance genes to other bacteria by horizontal gene transfer, such as conjugation.
Patients do not become antibiotic-resistant
It is the bacteria that become resistant, not the patient. A patient may carry resistant bacteria, but their own body cells have not “adapted” to the antibiotic.
Antibiotic use creates selection pressure, meaning it changes which variants are most likely to survive and reproduce. Hospitals apply strong selection pressure because antibiotics are used frequently and vulnerable patients may carry pathogens for longer.
Explaining the rise of resistant bacteria
A patient is treated with an antibiotic. After several days, the infection contains a higher proportion of resistant bacteria.
- Start with variation: before treatment, the bacterial population contains both susceptible and resistant bacteria.
- Apply selection pressure: the antibiotic kills or inhibits susceptible bacteria more effectively than resistant bacteria.
- Predict the population change: resistant bacteria survive, reproduce by binary fission, and become a larger proportion of the population.
Infection-control measures
Hand hygiene and personal protective equipment
Hand hygiene includes washing hands with soap and water or using alcohol hand gel where appropriate. It reduces transfer of pathogens between patients, staff and surfaces.
Personal protective equipment (PPE) includes gloves, aprons, masks and eye protection. PPE acts as a barrier, but it only works if used and removed correctly; contaminated gloves can still spread pathogens.
Aseptic technique
Aseptic technique means using procedures that prevent contamination by microorganisms. This includes sterile equipment, disinfected skin, minimal touching of key parts, and careful handling of wounds, cannulas and catheters.
Think barrier plus behaviour
Good infection control is not just “killing bacteria”. It also means stopping pathogens moving from one place to another.
Sterilisation, disinfection and antisepsis
Cleaning terms
Sterilisation kills or removes all microorganisms, including spores. Disinfection reduces microorganisms on non-living surfaces to a safer level. Antisepsis reduces microorganisms on living tissue, such as skin.
Examples include autoclaving surgical instruments, disinfecting bed rails, using antiseptic skin preparation before inserting a cannula, and safely disposing of sharps.
Isolation, screening and cohorting
Patients with highly transmissible infections may be isolated in single rooms. Cohorting means grouping patients with the same infection together, so staff can reduce spread to uninfected patients.
Hospitals may screen patients for MRSA carriage, especially before surgery. Some carriers may be given decolonisation treatment, such as antiseptic body wash or nasal ointment, to reduce the chance of infection and transmission.
Antibiotic stewardship
Antibiotic stewardship means using antibiotics responsibly. This includes prescribing only when needed, choosing a narrow-spectrum antibiotic where possible, using the correct dose and duration, and reviewing treatment when culture results are available.
This reduces unnecessary selection pressure and helps slow the spread of resistance.
Interpreting infection data
Hospitals monitor infection data to evaluate whether control measures are working. Two useful terms are incidence, the number of new cases in a time period, and prevalence, the total number of existing cases at a particular time.
In hospital data, infection rates are often standardised using patient-days. One patient-day means one patient in hospital for one day.
infection rate=number of new infectionspatient-days×1000\text{infection rate} = \frac{\text{number of new infections}}{\text{patient-days}} \times 1000infection rate=patient-daysnumber of new infections×1000Calculating an infection rate reduction
A ward records 18 new infections in 6000 patient-days before a new hand-hygiene programme. Afterwards, it records 7 new infections in 5000 patient-days.
- Calculate the original rate:
So the original infection rate is 3.0 infections per 1000 patient-days.
- Calculate the new rate:
So the new infection rate is 1.4 infections per 1000 patient-days.
- Calculate the percentage reduction:
The infection rate decreased by 53.3%.
Be careful: a fall in infection rate after an intervention is useful evidence, but it does not prove the intervention was the only cause. You should consider sample size, changes in patient risk, changes in testing, and whether staff followed the procedure consistently.
Practical link: antimicrobial testing
You may meet practical data from agar plates testing antimicrobial substances. A sterile agar plate is inoculated with a bacterial “lawn”, antimicrobial discs are added, and the plate is incubated. A clear zone of inhibition shows where bacterial growth has been prevented.
Important control variables include the bacterial strain, inoculum volume, antimicrobial concentration, disc size, agar depth, incubation time and temperature. In school laboratories, plates are incubated at lower temperatures than body temperature to reduce the risk of growing human pathogens.
Bigger zone, stronger conclusion?
A larger zone of inhibition suggests greater antimicrobial effect under those conditions, but it is not a perfect measure of effectiveness in the body because diffusion through agar also affects zone size.
In the exam
- Link each control measure to the part of the chain of infection it breaks, rather than listing measures randomly.
- For antibiotic resistance, describe selection acting on pre-existing resistant bacteria; do not say the bacteria “try to become resistant”.
- When using infection-rate data, calculate standardised rates first before comparing wards or time periods.
Check yourself
- Why are catheterised patients at increased risk of hospital-acquired infection?
- How does antibiotic use increase the proportion of resistant bacteria in a population?
- Why might soap and water be required during a C. difficile outbreak?
