AKT · Infections/Key pathogens & antimicrobials

Gram-negative & anaerobic organisms

Gram-negative rods/cocci + anaerobes — LPS endotoxin drives Gram-negative sepsis

Overview

A grouping of the high-yield Gram-negatives and anaerobes tested by context. E. coli — commonest cause of UTI and Gram-negative sepsis; O157:H7 (Shiga toxin) → HUS (avoid antibiotics). Pseudomonas aeruginosa — burns, hot-tub folliculitis, otitis externa, cystic fibrosis, blue-green pus; needs anti-pseudomonal cover (pip-tazobactam, ceftazidime, ciprofloxacin). Klebsiella — cavitating pneumonia in alcoholics, UTI. Neisseria meningitidis (Gram-negative diplococcus) — meningococcal sepsis. Gram-negative sepsis is driven by lipopolysaccharide (LPS) endotoxin. Anaerobes (Bacteroides, Clostridia) → intra-abdominal/aspiration infection, gas gangrene — covered by metronidazole.

Recognise

E. coli: dysuria/UTI, ascending pyelonephritis, Gram-negative bacteraemia; O157 → bloody diarrhoea + HUS in children (do NOT give antibiotics)
Pseudomonas: hot-tub folliculitis, otitis externa (esp. diabetics — malignant OE), CF/bronchiectasis exacerbations, burns turning blue-green (pyocyanin)
Klebsiella: aspiration/cavitating pneumonia + 'redcurrant jelly' sputum in alcoholics; Neisseria meningitidis: non-blanching rash + meningism

Red flags

Gram-negative sepsis → LPS-driven shock; Sepsis Six + broad-spectrum cover, source control
E. coli O157 → do NOT give antibiotics (increases HUS risk); meningococcal sepsis → benzylpenicillin + notify

Differentials & how to tell them apart

Gram-positive infectioncocci on Gram stain; exotoxin-mediated rather than LPS-endotoxin sepsis

Candidayeast; line/device or immunocompromised host, not a Gram-negative rod

Viral gastroenteritisnorovirus/rotavirus — watery, self-limiting, no blood, no bacterial culture

Investigations

Culture (urine, blood, sputum, pus); Gram stain (pink rods/diplococci); stool culture + Shiga-toxin testing for O157; anti-pseudomonal sensitivities; lactate/blood cultures in sepsis.

Management

Empirical Gram-negative cover (local policy) + anti-pseudomonal agent if Pseudomonas; metronidazole for anaerobes; NO antibiotics for O157

1Gram stain + culture to identify the organism and sensitivities; start empirical cover per local policy and de-escalate. Add anti-pseudomonal cover if Pseudomonas is likely (CF, burns, neutropenia).Gate: E. coli O157:H7 (Shiga toxin) → do NOT prescribe antibiotics or antimotility drugs — they increase the risk of haemolytic-uraemic syndrome.
2Source control (relieve obstruction, drain, remove device); metronidazole for anaerobic/mixed intra-abdominal or aspiration infection; notify meningococcal disease.

Co-amoxiclav / gentamicin / cephalosporins — empirical Gram-negative cover (per local policy); gentamicin for Gram-negative sepsis

Anti-pseudomonal agent — piperacillin-tazobactam, ceftazidime, ciprofloxacin, meropenem — Pseudomonas needs specific cover

Metronidazole — anaerobic cover (intra-abdominal, aspiration, C. difficile, gas gangrene)

Supportive only for E. coli O157 — antibiotics and antimotility agents INCREASE HUS risk — avoid

Key points

Gram-negative sepsis = LPS ENDOTOXIN (vs Gram-positive exotoxins). Context forks: UTI/sepsis → E. coli; blue-green pus/hot-tub/CF/burns → Pseudomonas (needs specific cover); cavitating pneumonia in an alcoholic → Klebsiella; non-blanching rash → Neisseria meningitidis; O157 bloody diarrhoea → supportive, NO antibiotics (HUS). Metronidazole covers anaerobes.

Monitor & prognosis

Cultures/sensitivities, lactate and organ function in sepsis, renal function (HUS) in O157.

Good with prompt appropriate antibiotics + source control; Gram-negative septic shock and HUS carry high morbidity.

Source: NICE sepsis NG51; UKHSA; BNF