What Is Spontaneous Bacterial Peritonitis?

Spontaneous bacterial peritonitis (SBP) is an infection of the fluid that accumulates in the abdomen of people with advanced liver disease, and it develops without any obvious source like a ruptured organ or surgical wound. It occurs almost exclusively in patients who already have cirrhosis and ascites, the pathologic buildup of fluid in the peritoneal cavity driven by portal hypertension and hormonal imbalances. SBP carries significant mortality even with treatment, making early recognition and prompt intervention critical.

Why the Infection Happens in the First Place

The story of SBP begins in the gut. In a healthy person, the intestinal lining acts as a selective barrier, keeping bacteria confined to the bowel. In advanced cirrhosis, that barrier breaks down through a cascade of problems. Portal hypertension causes blood to back up in the vessels supplying the intestines, leading to swelling of the gut wall and increased permeability. At the same time, the liver’s ability to clear bacteria from the bloodstream deteriorates as the disease worsens. The immune system itself becomes paradoxically impaired: systemic inflammation ramps up while the body’s ability to fight off actual pathogens declines, a state sometimes called cirrhosis-associated immune dysfunction.¹

The result is bacterial translocation, where live bacteria cross from the intestinal lumen into the mesenteric lymph nodes and eventually reach the bloodstream or the ascitic fluid directly. An animal study using a cirrhosis model found that translocation occurred in roughly four out of five cirrhotic rats, compared to fewer than one in twenty healthy controls, with gram-negative gut organisms like E. coli being the primary culprits.² While rat models are not a perfect stand-in for human disease, this finding aligned with what clinicians were seeing at the bedside: the bacteria cultured from infected ascitic fluid were overwhelmingly the same species that live in the human gut.

The composition of the gut microbiome itself shifts in cirrhosis. Patients with SBP show an overgrowth of potentially harmful bacteria like Gammaproteobacteria, Klebsiella, and Acinetobacter, while protective species such as Faecalibacterium and Lactobacillus become depleted.³ This imbalance, called dysbiosis, further damages the intestinal barrier and feeds a cycle where more harmful bacteria gain access to the peritoneal cavity. Bile acid disruption in cirrhosis makes things worse by altering the environment in which gut bacteria thrive, promoting the overgrowth of pathogenic species at the expense of protective ones.⁴

How Ascites Sets the Stage

SBP cannot happen without ascites, and ascites itself reflects a body fighting a losing battle over fluid balance. In cirrhosis, scarring of the liver raises pressure in the portal vein system, causing blood vessels in the gut to dilate dramatically. This dilation, driven largely by excess nitric oxide in the splanchnic circulation, tricks the body into sensing that its arteries are underfilled.⁵ The kidneys respond by holding onto sodium and water aggressively, hormonal systems like the renin-angiotensin-aldosterone axis go into overdrive, and the excess fluid leaks into the peritoneal cavity through overwhelmed lymphatic channels.⁶

That pooled ascitic fluid becomes a warm, protein-poor medium where bacteria can multiply with relatively little immune resistance. When the protein content of ascitic fluid drops below a certain level, the fluid lacks the opsonins and complement factors that normally help immune cells kill bacteria. This is one reason that very low ascitic fluid protein levels are used as a marker for who is at highest risk of developing SBP and who should receive preventive antibiotics.

Symptoms That Are Easy to Miss

One of the most dangerous aspects of SBP is that its symptoms can be subtle, especially in patients who already feel terrible from advanced liver disease. Fever, which most people would expect from a serious infection, appears in fewer than half of episodes. In one clinical study, the most common findings during SBP episodes were jaundice and abdominal tenderness, each occurring in just over half of cases, followed by hepatic encephalopathy in about half and abdominal pain in roughly 45%.⁷ Fatigue was also common. Many of these symptoms overlap with what a cirrhotic patient experiences on a bad day even without infection, which is why clinical guidelines recommend a low threshold for testing.

The practical takeaway: any patient with cirrhosis and ascites who develops new abdominal pain, confusion, fever, or unexplained worsening of their condition should have their ascitic fluid sampled promptly. Waiting for a classic picture of high fever and a rigid abdomen means missing cases. Some patients present with nothing more than a subtle change in mental status or new-onset kidney problems.

How SBP Is Diagnosed

Diagnosis comes down to a procedure called paracentesis, where a needle is inserted into the abdomen to withdraw a sample of ascitic fluid. The key laboratory finding is a neutrophil count above 250 cells per cubic millimeter in the fluid.⁸ That number alone is enough to begin treatment, even before culture results come back.⁹ A higher threshold of 500 cells per cubic millimeter carries greater sensitivity for detecting true infection.¹⁰

Cultures of ascitic fluid are important for guiding antibiotic therapy, but they come back negative in a large proportion of cases. This entity, called culture-negative neutrocytic ascites, behaves clinically the same as culture-positive SBP: the symptoms, the severity, and the mortality are comparable. An early study found that mortality in culture-negative cases was about 50%, in the same range as culture-positive SBP, leading to the recommendation that all cases meeting the neutrophil threshold be treated with antibiotics regardless of culture results.¹¹

One practical challenge in busy emergency settings is the turnaround time for a formal cell count. Leukocyte esterase reagent strips, the same dipsticks used for urine testing, have been studied as a rapid bedside alternative. Multiple studies have found sensitivity and specificity above 95% when using an appropriate color-change threshold, allowing clinicians to start antibiotics within minutes rather than hours.¹²¹³ Performance does vary by brand, however, so not all dipsticks are equally reliable for this purpose.¹⁴

Distinguishing SBP from Secondary Peritonitis

Not every infected ascites is SBP. Secondary peritonitis, caused by a perforated organ or an abscess leaking into the abdomen, requires surgery and carries a different management pathway. Telling the two apart matters enormously. In secondary peritonitis, the ascitic fluid tends to show multiple organisms on culture, extremely high neutrophil counts, elevated protein and LDH levels, and low glucose, in contrast to the typically single-organism or culture-negative pattern seen in SBP.¹⁵ A multicentre retrospective study confirmed that microbiological characteristics of the ascitic fluid, combined with severity-of-illness markers, are the strongest predictors for distinguishing the two conditions.¹⁶ When clinicians suspect secondary peritonitis, imaging and sometimes surgery are needed in addition to antibiotics.

Which Bacteria Cause SBP and Why Resistance Matters

Traditionally, SBP has been viewed as a gram-negative infection, and for good reason: E. coli remains the single most common organism isolated, accounting for about a quarter of cultured episodes. Klebsiella pneumoniae follows at around 12%, and Enterococcus faecium at roughly 10%.¹⁷ But the microbiological landscape is shifting, and that shift has direct consequences for treatment.

In recent years, gram-positive organisms and multidrug-resistant (MDR) bacteria have become increasingly common in SBP, particularly in hospital-acquired cases. One study found that MDR bacteria were isolated in nearly half of all SBP samples, with resistance to third-generation cephalosporins reaching about 39% and quinolone resistance at about 26%.¹⁸ The problem is especially pronounced in nosocomial SBP, where MDR rates are roughly double those seen in community-acquired infections, and resistance extends to piperacillin-tazobactam and even carbapenems in a substantial minority of cases.¹⁹ Infections with MDR organisms have been independently linked to higher in-hospital mortality and more complications.²⁰

This rising resistance has created a tension in clinical practice. Standard empiric therapy may fail more often in hospitalized patients, pushing some centers toward broader-spectrum antibiotics as first-line treatment for nosocomial cases while preserving narrower regimens for community-acquired SBP.

Treatment With Antibiotics and Albumin

The backbone of SBP treatment is intravenous antibiotics started as soon as the diagnosis is suspected, without waiting for culture results. Third-generation cephalosporins like cefotaxime and ceftriaxone have been the standard for decades. A randomized trial comparing cefotaxime, ceftriaxone, and ciprofloxacin found no significant difference in efficacy among the three, and all remained effective as initial therapy for community-acquired SBP.²¹ However, response rates are not as reassuring as one might hope. One study found that only about 57% of patients responded to initial treatment with third-generation cephalosporins, leaving a large minority needing a switch to broader coverage.²²

Alongside antibiotics, intravenous albumin plays a crucial role. SBP can trigger a dangerous drop in circulating blood volume, leading to kidney failure, a complication that substantially increases the chance of dying. A landmark trial published in the New England Journal of Medicine showed that adding albumin to antibiotic therapy reduced the rate of kidney impairment from about 33% to 10%.²³ The benefit appears greatest in patients who are already at high risk for kidney problems, specifically those with elevated bilirubin, elevated blood urea nitrogen, or elevated creatinine at the time of diagnosis.²⁴ Whether albumin helps patients with milder SBP and normal kidney function to the same degree remains less clear, and practice varies among centers.

Preventing a First or Second Episode

Because SBP recurs frequently and each episode carries a real risk of death, preventive antibiotics are a mainstay for eligible patients. The two main contexts are primary prophylaxis, given to patients who have never had SBP but are at high risk due to low ascitic fluid protein, advanced liver disease scores, or kidney dysfunction, and secondary prophylaxis, given after a patient survives a first episode.

Norfloxacin, a fluoroquinolone taken once daily by mouth, has been the most studied prophylactic agent. A systematic review and meta-analysis of 12 randomized controlled trials found that norfloxacin cut the rate of SBP by roughly fivefold compared to placebo and was also associated with lower mortality.²⁵ The concern with long-term fluoroquinolone use, though, is that it promotes the very drug-resistant organisms that make SBP harder to treat when it does occur.

Rifaximin, a gut-targeted antibiotic with minimal systemic absorption, has emerged as an alternative. A meta-analysis comparing it to norfloxacin found comparable effects on SBP prevention and survival, with rifaximin showing fewer adverse events and possibly better prevention of recurrence.²⁶ Rifaximin also appears to lower endotoxin levels in the blood and reduce neutrophil counts in ascitic fluid, suggesting it may address the underlying bacterial translocation rather than just suppressing infection.²⁷ An attractive practical advantage is that many cirrhotic patients already take rifaximin for hepatic encephalopathy, so it can serve dual purposes with a single pill.²⁸

Prognosis and the Case for Transplant Evaluation

A first episode of SBP is a turning point in the trajectory of liver disease. Even with appropriate treatment, about a third of patients die within 30 days. Longer-term survival is grim: one study reported that 78% of patients who developed SBP were dead within a year.²⁹ Another found a one-year survival rate of 46% in patients who were potential transplant candidates, but that number dropped sharply in patients with the most severe liver dysfunction.³⁰

The development of acute-on-chronic liver failure (ACLF) in the setting of SBP is a particularly ominous sign. In one cohort, nearly 60% of patients with SBP met criteria for ACLF, and its presence was independently associated with lower survival at both 28 and 90 days.³¹ SBP is, along with pneumonia, one of the infections most frequently linked to triggering ACLF in cirrhotic patients.³²

Because of these numbers, a first episode of SBP is widely regarded as an indication to begin evaluating a patient for liver transplantation. Transplant remains the only definitive treatment for the underlying liver disease, and survival after transplant far exceeds what can be achieved with medical management alone. The challenge is that many patients with SBP are poor transplant candidates due to age, ongoing alcohol use, or other medical conditions, which means a significant number will never receive that option.

The Role of Beta-Blockers

Nonselective beta-blockers have long been used in cirrhosis to lower portal pressure and prevent variceal bleeding, but their role in patients with SBP and ascites has been debated. Some earlier data raised concern that beta-blockers could be harmful in the sickest patients by lowering blood pressure and cardiac output in people who already have a precarious circulation. More recent evidence, however, points in the opposite direction for many patients. A propensity-matched study found that patients with SBP who were on beta-blocker therapy had lower mortality than those who were not, although the beta-blocker group did show a higher incidence of acute kidney injury.³³ The kidney risk means these medications need to be managed carefully, with dose adjustments or temporary discontinuation if blood pressure drops too low or kidney function deteriorates. Still, the overall survival benefit in this study suggests that beta-blockers should not be reflexively stopped when SBP develops, a shift from older, more cautious recommendations.

Why the Gut Microbiome Is Becoming a Research Focus

Researchers are increasingly looking at the gut microbiome not just as the source of bacteria in SBP, but as a potential therapeutic target. The finding that patients with SBP have distinct microbial signatures, with enrichment of Proteobacteria and depletion of protective anaerobes, has raised interest in whether restoring a healthier microbial community could reduce the risk of infection.³⁴ Rifaximin’s mechanism of action may partly work through this pathway, reshaping the gut flora rather than simply killing bacteria. Fecal microbiota transplantation has been studied in other contexts related to cirrhosis, though evidence specific to SBP prevention is still in its early stages. The appeal of microbiome-based approaches is that they could, in theory, address the root cause of bacterial translocation rather than relying on lifelong antibiotic prophylaxis, which itself contributes to resistance.