What type of bacteria, gram-positive or gram-negative, should be suspected in cases of bacteremia?

From the Guidelines

Both gram-positive and gram-negative bacteria should be suspected in cases of bacteremia, as either type can cause bloodstream infections. The specific suspicion depends on the clinical context, patient factors, and source of infection. Gram-positive organisms like Staphylococcus aureus, coagulase-negative staphylococci, and Enterococcus species are common causes of bacteremia, particularly in catheter-related infections, endocarditis, and skin/soft tissue infections. Gram-negative bacteria such as Escherichia coli, Klebsiella species, and Pseudomonas aeruginosa frequently cause bacteremia in urinary tract infections, intra-abdominal infections, and healthcare-associated pneumonia.

• Initial empiric antibiotic therapy should cover both gram-positive and gram-negative organisms until culture results are available, typically with a combination of antibiotics like vancomycin plus an extended-spectrum beta-lactam or carbapenem, as recommended by the Infectious Diseases Society of America 1.
• The distinction matters because gram-positive and gram-negative bacteria have different cell wall structures, which affects their susceptibility to various antibiotics.
• Blood cultures are essential for definitive identification and targeted therapy, which should be narrowed once the causative organism and its susceptibilities are known, as emphasized in the management of intra-abdominal infections 1.
• The choice of empiric antibiotic regimens in patients with bacteremia should be based on the clinical condition of the patients, the individual risk for infection by resistant pathogens, and the local resistance epidemiology, as highlighted in the guidelines for management of intra-abdominal infections 1.
• In patients with suspected bacteremia, empiric antimicrobial therapy should be started as soon as possible, and the selection of appropriate empiric antibiotic therapy is critical for preventing unnecessary morbidity and mortality, as noted in the strategies to reduce curative antibiotic therapy in intensive care units 1.

From the FDA Drug Label

Meropenem has been shown to be active against most isolates of the following microorganisms, both in vitro and in clinical infections ... Gram-positive bacteria ... Gram-negative bacteria ... The FDA drug label does not directly answer what type of bacteria should be suspected in cases of bacteremia, but it does list the types of gram-positive and gram-negative bacteria that meropenem is active against.

• Gram-positive bacteria include:
  • Enterococcus faecalis
  • Staphylococcus aureus
  • Streptococcus agalactiae
  • Streptococcus pneumoniae
  • Streptococcus pyogenes
  • Viridans group streptococci
• Gram-negative bacteria include:
  • Escherichia coli
  • Haemophilus influenzae
  • Klebsiella pneumoniae
  • Neisseria meningitidis
  • Proteus mirabilis
  • Pseudomonas aeruginosa 2

From the Research

Bacteremia Representation

In cases of bacteremia, the type of bacteria suspected can be either gram-positive or gram-negative. The evidence suggests that both types of bacteria can cause bacteremia, but the distribution and antimicrobial resistance patterns vary.

Gram-Positive Bacteria

• Coagulase-negative staphylococci (CoNS) are the most frequently isolated gram-positive bacteria in blood cultures, accounting for 40.23% of cases 3.
• Staphylococcus aureus is another common gram-positive bacterium, with a detection rate of 5.15% 3.
• Enterococci, including Enterococcus faecalis and Enterococcus faecium, are also significant gram-positive pathogens, with intrinsic resistance to antibiotics 4.

Gram-Negative Bacteria

• Escherichia coli (E. coli) is the most frequent gram-negative bacterium, followed by Klebsiella pneumoniae, Pseudomonas aeruginosa, and other Enterobacteriaceae species 5, 6, 7.
• The urinary tract is the most common source of gram-negative bacteremia, but in some cases, the original source cannot be identified 7.

Antimicrobial Resistance

• Gram-positive bacteria, such as Staphylococcus aureus and CoNS, exhibit resistance to various antibiotics, including methicillin and vancomycin 3, 4.
• Gram-negative bacteria, such as E. coli and Klebsiella pneumoniae, also show resistance to antibiotics, including ampicillin and third-generation cephalosporins 6, 3.
• The choice of empiric antibiotic therapy should be guided by the local antimicrobial resistance patterns and the severity of the underlying disease 5, 4.