How microorganisms cause harm

Spoilage and illness are different outcomes of microbial activity. Decomposition may release odorous compounds, gas or coloured growth. Some spoilage organisms are not typical causes of illness, while some moulds produce mycotoxins. A visible change is a reason for appropriate disposal, but normal appearance is not evidence that a food is safe.

Foodborne intoxication follows exposure to toxin already formed in food. Staphylococcal enterotoxins and the emetic toxin of Bacillus cereus can cause rapid vomiting, often within hours. Not every toxin behaves identically: botulism has a different clinical pattern, and other hazards have different onset times. Infection follows ingestion of an organism that establishes itself in the host. Salmonella symptoms commonly begin from six hours to six days; Campylobacter usually from two to five days. These intervals guide investigation, but overlap prevents diagnosis by the clock alone.

Heating kills many organisms, yet some toxins are heat-stable and some spores survive ordinary cooking. Refrigeration slows many microbes rather than sterilising food; Listeria can grow at refrigeration temperatures. Avoid trying to rescue a suspect dish by reheating. Prevention combines clean handling, separation of raw and cooked items, thorough cooking, suitable holding temperature and safe water. Food-safety instructions vary with product and local policy, and retained outbreak samples should be handled according to public-health advice.

Microbial harm depends on strain, site and host defences. Colonisation means presence without disease at that site; opportunistic infection can occur when circumstances change. Some E. coli strains are normal colon inhabitants, while particular strains or movement to another site can cause illness. Bladder infection is assessed from symptoms and appropriate tests, not from a universal assumption that all urine is microbe-free. The organism need not change species to have a different effect.

Staphylococcus epidermidis lives harmlessly on skin and becomes one of the commonest causes of bloodstream infection when it forms a biofilm on an intravenous catheter or a prosthetic heart valve. Streptococcus pneumoniae is carried in the nasopharynx of many healthy children, yet causes otitis media, pneumonia and meningitis when it spreads to the middle ear, the lung or the meninges. Candida albicans is a normal resident of the mouth, gut and vagina; broad-spectrum antibiotics remove the bacteria that usually restrain it, and in a patient with neutropenia or a long intensive-care stay it can invade the bloodstream. Clostridioides difficile behaves in the same way: the organism may be present without symptoms, and disease appears when antibiotics disturb the gut flora and the bacterium produces its toxins.

Four mechanisms explain these changes of role: a barrier is breached, as with a wound, a catheter or surgery; the organism translocates from its normal site to a sterile one; the host defences fall; or a device gives the organism a surface on which to build a biofilm. The nurse’s questions about a positive culture are always the same: what site was sampled, does the patient have signs of infection at that site, what device is in place, and what has changed in the patient’s defences? A positive swab from a chronic ulcer is not automatically an infection, and treating it with antibiotics because the laboratory reported a name is one of the commonest errors in ward practice. Hand hygiene, careful device care, aseptic technique and the prudent use of antibiotics are the interventions that keep a colonising organism in its place.

Transmission explains which preventive action can interrupt exposure. Malaria is usually spread by infected female Anopheles mosquitoes; ordinary contact does not spread it, although rare blood-related or congenital transmission occurs. Bed nets and other locally indicated vector-control measures reduce exposure. Sleeping sickness is spread mainly by tsetse flies. The gambiense form is generally slower than rhodesiense, but both progress from an early stage to central nervous system involvement; neurological disease is not automatically present from the start. Toxoplasmosis may follow ingestion of tissue cysts in undercooked meat or oocysts from contaminated soil or water, and infection can cross the placenta.

Histoplasmosis is not contagious between people: its spores are inhaled from soil enriched with bird or bat droppings, for example while cleaning a chicken coop or exploring a cave, and the illness is usually mild, though it can disseminate in an immunocompromised patient. Ringworm spreads by direct skin contact and by shared combs, towels, mats and shoes, so an outbreak in a school or barracks is controlled by treating cases and cleaning shared objects, not by isolating one child. Candidiasis is mostly endogenous: it comes from the patient’s own flora when antibiotics, diabetes, steroids or a damaged immune system remove the usual restraint.

Influenza spreads in respiratory droplets and on hands, and its surface antigens drift from season to season, so last year’s immunity may not protect; vaccination must be repeated and offered to health workers as well as patients. Yellow fever is transmitted by Aedes mosquitoes, particularly Aedes aegypti, which breed in domestic water containers and bite by day; a safe vaccine remains the mainstay of control. HIV is transmitted sexually, through infected blood and needles, and from mother to child in pregnancy, birth or breastfeeding; it destroys CD4 T lymphocytes over years, so the nurse follows the CD4 count and viral load and anticipates the opportunistic infections that appear as the count falls. Reducing transmission is therefore always specific: a net for malaria, gloves and a safe water supply for toxoplasmosis, a vaccine for yellow fever, sterile needles and never recapping for HIV, and ventilation with a mask for influenza.