Gram-Negative Bacteria: MRCOG Part 1 questions and key facts

The clinical presentation of acute onset bloody diarrhoea, abdominal cramps, and fever, particularly following the consumption of undercooked poultry, is highly suggestive of bacterial gastroenteritis caused by Campylobacter species, most commonly Campylobacter jejuni. Campylobacter is a Gram-negative, motile, curved or spiral-shaped rod and is one of the leading causes of bacterial foodborne illness globally. Poultry is a major reservoir for Campylobacter.

While other bacteria can cause similar symptoms, Campylobacter fits the described scenario best. Salmonella species also cause gastroenteritis often linked to poultry and eggs, presenting with diarrhoea (which can sometimes be bloody), fever, and cramps, but Campylobacter is frequently cited as the most common bacterial cause of diarrhoea in many developed countries, often associated with bloody stools. Shigella species cause bacillary dysentery characterised by bloody diarrhoea and cramps, but transmission is typically faecal-oral, often person-to-person or via contaminated water/food, and less classically associated with poultry. Listeria monocytogenes (a Gram-positive rod) usually causes invasive disease (sepsis, meningitis) rather than primarily gastroenteritis, and is linked to different food sources (e.g., deli meats, soft cheeses). Helicobacter pylori causes gastritis and peptic ulcers, not acute infectious bloody diarrhoea.

Early-onset neonatal sepsis (EONS), defined in UK guidance as infection within the first 72 hours of life, is usually caused by bacteria acquired from the maternal genital tract before or during labour. In high-income countries such as the UK and USA, Group B Streptococcus (GBS, Streptococcus agalactiae) and Escherichia coli together account for most cases. GBS predominates in term infants, while E. coli is the commonest pathogen in preterm infants and carries the highest mortality. Both organisms colonise the maternal vagina and rectum and can ascend into the amniotic cavity or be acquired during passage through the birth canal. Presentation is non-specific: fever or hypothermia, lethargy, poor feeding, respiratory distress and poor tone.

Listeria monocytogenes is an important but much less common cause of EONS; it is usually acquired transplacentally after maternal food-borne infection and can cause sepsis and meningitis. Staphylococcus aureus is a more common cause of late-onset neonatal sepsis (after 72 hours), often healthcare-associated, for example via central lines or skin breakdown. Pseudomonas aeruginosa is an opportunistic pathogen of preterm or hospitalised neonates in intensive care rather than a cause of EONS in term infants. Haemophilus influenzae is an uncommon cause of EONS; neonatal cases are mostly due to non-typeable (unencapsulated) strains, often in preterm infants.

The clinical presentation of fever, headache, and a rapidly spreading non-blanching purpuric rash is highly suggestive of meningococcal septicaemia, caused by Neisseria meningitidis. This gram-negative diplococcus colonises the nasopharynx and can invade the bloodstream, leading to sepsis and/or meningitis. The characteristic non-blanching rash (petechiae and purpura) is caused by endotoxin release leading to disseminated intravascular coagulation (DIC) and damage to small blood vessels. Meningococcal disease can progress very rapidly and is a medical emergency requiring immediate antibiotic treatment. University students, particularly those living in shared accommodation like halls of residence, are at increased risk.

While Streptococcus pneumoniae is a common cause of bacterial meningitis, it is less frequently associated with the rapidly progressive purpuric rash seen in meningococcal disease. Staphylococcus aureus can cause sepsis with skin manifestations (e.g., pustules, cellulitis, or toxic shock syndrome rash), but the classic fulminant purpuric rash is less typical. Haemophilus influenzae type b meningitis and sepsis are now rare in immunised populations. Escherichia coli is a common cause of sepsis, particularly urinary tract or abdominal sourced, and neonatal meningitis, but rarely presents this way in an immunocompetent student.