Campylobacter jejuni: Gastroenteritis, Guillain-Barré Syndrome, and Laboratory Diagnosis

A young adult develops several days of foul-smelling, sometimes bloody diarrhea with cramping and fever after a barbecue where the chicken was undercooked. The diarrhea settles on its own within a week. But a couple of weeks later, something alarming happens: weakness starts in the legs and creeps upward, and the person struggles to walk. The gut infection has triggered Guillain-Barré syndrome, and behind both events is the same organism: Campylobacter jejuni.

Campylobacter is the most common bacterial cause of diarrhea in the world, usually a self-limited illness from undercooked poultry. What makes it stand out is what can follow it: in a small number of people, the immune response to the infection turns against the body's own nerves. This article is about how Campylobacter causes gut disease, how it triggers this nerve complication, and how the laboratory identifies it.

Introduction

Campylobacter jejuni is a curved Gram-negative rod and the most common bacterial cause of gastroenteritis worldwide. It typically causes diarrhea (often bloody) and enterocolitis, especially in children, from eating undercooked poultry or drinking contaminated water or milk. Most infections are self-limited, but Campylobacter is important out of proportion to the usual mild illness for two reasons: it can trigger serious autoimmune complications, above all Guillain-Barré syndrome, and drug-resistant strains are a growing concern (the CDC lists drug-resistant Campylobacter as a serious threat).

A related species, Campylobacter fetus, causes systemic infection (bloodstream infection) rather than gut disease, mainly in the elderly and debilitated.

What Campylobacter jejuni looks like, and what that tells you

A few features identify Campylobacter, and two of them are genuinely distinctive.

Curved shape. C. jejuni is a Gram-negative rod that is curved, comma-shaped, or S-shaped, and when two cells join, they can look like a "seagull wing" or "gull's wing." This shape on a stained smear from a stool culture is a useful early clue. It shares the curved shape with Vibrio, but the two are separated easily by the tests below.

Figure: Campylobacter with amphitrichous (bipolar) flagella

It is microaerophilic. Campylobacter grows best in a low-oxygen atmosphere (around 5% oxygen), not the 21% in room air, and it also needs added carbon dioxide. This is why it needs a special gas environment to culture and is missed on ordinary aerobic plates. The oxygen requirements of bacteria are covered separately.

It grows at 42°C (the thermophilic clue). Unlike most gut organisms, which prefer body temperature, C. jejuni grows best at 42°C. Laboratories use this: incubating a stool culture at 42°C favors Campylobacter and suppresses much of the normal flora. This is one of its most testable features. (By contrast, C. fetus grows at 25°C but not 42°C, the reverse pattern.)

It is oxidase-positive and catalase-positive. The oxidase-positive result separates it from the Enterobacteriaceae (which are oxidase-negative). It also has flagella at both ends (amphitrichous/bipolar), giving it a fast, darting, corkscrew motility.

Hold these together: a curved, oxidase-positive, microaerophilic Gram-negative rod that grows at 42°C is Campylobacter jejuni.

How Campylobacter spreads

Campylobacter lives in the gut of many animals, especially poultry (chickens), but also cattle and dogs. People get infected by the fecal-oral route, mostly through undercooked poultry, unpasteurized milk, or water contaminated with animal feces. Undercooked chicken is the classic source. Person-to-person spread happens but is less common. The infectious dose is relatively low.

Campylobacteriosis: the gut illness

After 2 to 5 days, infection causes watery, foul-smelling diarrhea that may become bloody, with cramping abdominal pain (which can be severe enough to mimic appendicitis), nausea, and fever. It is usually self-limited, lasting about a week. Some infected people have no symptoms at all. Because it usually resolves on its own, most cases need only rehydration, not antibiotics.

The autoimmune complications

Campylobacter is unusual among gut pathogens because the immune response to it can turn against the body's own tissues after the infection itself has cleared. Two complications matter.

Guillain-Barré syndrome (GBS)

This is the most important complication to know. Guillain-Barré syndrome is an acute paralysis that ascends from the legs upward, and it is the most common cause of acute flaccid paralysis now that polio is rare. C. jejuni infection is the single most commonly identified trigger, preceding a significant share of cases.

Figure: Mechanism of Guillain-Barré syndrome

The mechanism is molecular mimicry, and it is worth understanding rather than memorizing. The sugar coating (lipooligosaccharide) on the surface of C. jejuni closely resembles molecules (gangliosides) on the surface of human peripheral nerves. When the immune system makes antibodies against the Campylobacter, those antibodies also cross-react with the nerves, because the two look alike.

The antibodies attack the peripheral nerves, damaging them and causing the weakness. So the paralysis is not caused by the bacteria directly; it is caused by the body's own antibodies attacking nerves that resemble the bacterium. This is why GBS appears a week or two after the diarrhea, once the antibody response has developed, and why treating the gut infection does not prevent it.

Reactive arthritis

Campylobacter can also trigger reactive arthritis: painful swelling of joints, often the knees and ankles, appearing after the infection. This is another post-infectious immune reaction. Several other gut and genital pathogens (Chlamydia trachomatis, Salmonella, Shigella, Yersinia) can trigger it too.

Virulence factors and how Campylobacter causes disease

C. jejuni causes gut disease through a combination of motility, invasion, and toxins.

Motility and shape. Its corkscrew shape and bipolar flagella let it drill through the mucus layer of the gut to reach the lining cells, an advantage in the thick mucus of the intestine. Motility is essential for it to establish infection.

Adhesion and invasion. Campylobacter attaches to and invades the cells lining the intestine, particularly in the lower small bowel and colon. This invasion damages the lining and provokes the inflammation that produces bloody, inflammatory diarrhea (pus and blood in the stool).

Toxins. It produces enterotoxins (which drive fluid secretion, contributing to watery diarrhea) and cytotoxins (which damage cells). A cytolethal distending toxin (CDT) interferes with the host cell cycle and contributes to cell damage.

The surface sugar that causes trouble later. The lipooligosaccharide on its surface is both a virulence factor and the molecule responsible for the molecular mimicry behind Guillain-Barré syndrome, as described above.

Putting it together

Campylobacter is swallowed, uses its corkscrew motility to penetrate the gut mucus, and invades the lining of the lower intestine. The invasion and its toxins damage the mucosa and trigger inflammation, producing the bloody, foul-smelling diarrhea of campylobacteriosis. In most people the infection clears in about a week. In a small number, the antibodies raised against the organism's surface sugar cross-react with peripheral nerves, causing Guillain-Barré syndrome after the gut illness has settled.

Identifying Campylobacter in the laboratory

Specimen. Fresh diarrheal stool, ideally containing the blood, pus, and mucus of inflammatory diarrhea. If it cannot be processed at once, it can be refrigerated for up to about 24 hours (the organism tolerates cold better than room temperature).

Microscopy. A stained smear or a wet mount can show the curved, S-shaped, or gull-wing organisms with fast darting motility, a useful early clue.

Culture, using its two special requirements. Campylobacter is grown by exploiting exactly the features that make it distinctive:

• Selective medium: a blood agar containing antibiotics (such as Skirrow's or Campylobacter-selective agar) to suppress the normal stool flora.
• Microaerophilic atmosphere: about 5% oxygen with added CO₂, not room air.
• 42°C incubation: favors C. jejuni and suppresses competitors.

A curved Gram-negative rod that grows under these conditions, and is oxidase-positive and catalase-positive, is Campylobacter.

Separating C. jejuni from C. fetus. The two are told apart by temperature and nalidixic acid: C. jejuni grows at 42°C and is nalidixic acid-sensitive; C. fetus grows at 25°C (not 42°C) and is nalidixic acid-resistant. So the gut pathogen (jejuni) likes it hot and is nalidixic-sensitive; the systemic one (fetus) is the opposite.

Treatment

Most campylobacteriosis is self-limited and needs only rehydration. When antibiotics are indicated (severe or prolonged illness, bloody diarrhea, or a vulnerable patient), a macrolide such as azithromycin is the usual drug of choice.

Fluoroquinolones were used but resistance is now common, so they are less reliable, which is why Campylobacter appears on drug-resistance threat lists. The choice should be guided by local resistance patterns. Specific doses and durations are clinical decisions and are not covered here.

How to remember

Curved, and it likes it hot. Campylobacter is a curved (comma or S-shaped, gull-wing) rod that grows at 42°C, hotter than body temperature. Picture it thriving in the warm gut of a chicken, its main source. Curved plus 42°C plus poultry ties three facts together.

Camp-y-lo-bacter and molecular mimicry. The one link to fix in memory: Campylobacter is the most common trigger of Guillain-Barré syndrome. Its surface sugar looks like the coating on your nerves, so antibodies against the bug attack the nerves. The paralysis comes weeks after the diarrhea, once the antibodies have built up. Gut bug, then nerve attack, by mistaken identity.

Two Campylobacters, opposite habits. C. jejuni (gut) grows at 42°C and is nalidixic-sensitive. C. fetus (blood, in the debilitated) grows at 25°C and is nalidixic-resistant. Hot-and-sensitive versus cool-and-resistant.

Microaerophilic: not too much air. Campylobacter wants about 5% oxygen, not room air. It will not grow on an ordinary aerobic plate. Picture it needing just a little air, which is why culture needs a special gas jar.

Key exam facts

Where students get confused

Campylobacter vs Vibrio (both curved). Both are curved, oxidase-positive Gram-negative rods. Separators: Campylobacter is microaerophilic and grows at 42°C; Vibrio is a facultative anaerobe that grows on ordinary media and on TCBS. Different growth needs, same shape.

How a gut infection causes paralysis. Guillain-Barré is not the bacteria invading nerves. It is the body's antibodies against Campylobacter cross-reacting with nerve gangliosides (molecular mimicry). That is why GBS appears a week or two after the diarrhea and why antibiotics for the gut infection do not prevent it.

C. jejuni* vs *C. fetus. The gut one (jejuni) grows at 42°C and is nalidixic-sensitive; the systemic one (fetus) grows at 25°C and is nalidixic-resistant. The older name "C. intestinalis" for the systemic organism is outdated; it is C. fetus.

Ciprofloxacin is no longer reliable. Older sources list fluoroquinolones as treatment, but resistance is now widespread, so a macrolide (azithromycin) is the better choice. This is a common outdated point.

Microaerophilic gets missed. Campylobacter will not grow on a standard aerobic plate; it needs about 5% oxygen. A "no growth" on ordinary culture does not rule it out if the right atmosphere was not used.

References

1. Altekruse, S. F., Stern, N. J., Fields, P. I., & Swerdlow, D. L. (1999). Campylobacter jejuni: an emerging foodborne pathogen. Emerging Infectious Diseases, 5(1), 28–35. https://doi.org/10.3201/eid0501.990104
2. Young, K. T., Davis, L. M., & DiRita, V. J. (2007). Campylobacter jejuni: molecular biology and pathogenesis. Nature Reviews Microbiology, 5(9), 665–679. https://doi.org/10.1038/nrmicro1718
3. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
4. Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.