Intrinsic Resistance Patterns

This section highlights the inherent resistance patterns of common bacterial species. Understanding these intrinsic resistance profiles is crucial for selecting appropriate antibiotics, interpreting susceptibility test results, and avoiding ineffective treatments

Intrinsic Resistance

Definition: Intrinsic resistance refers to the natural resistance of a bacterial species to a particular antibiotic or class of antibiotics. This resistance is inherent to the organism and is not acquired through mutation or horizontal gene transfer
Mechanisms: Intrinsic resistance can be due to various factors, including:
- Lack of Target: The organism may lack the target of the antibiotic
- Impermeability: The bacterial cell wall or membrane may be impermeable to the antibiotic
- Efflux Pumps: The organism may possess efflux pumps that actively transport the antibiotic out of the cell
- Enzymatic Inactivation: The organism may produce enzymes that inactivate the antibiotic
Importance: Knowing the intrinsic resistance patterns of common species allows for:
- Empiric Therapy: Selecting appropriate antibiotics for initial treatment before susceptibility test results are available
- Interpreting Susceptibility Results: Recognizing when an isolate is intrinsically resistant, even if the susceptibility test results suggest otherwise
- Avoiding Ineffective Treatments: Preventing the use of antibiotics that are known to be ineffective against a particular organism

Intrinsic Resistance Patterns of Common Species

Gram-Positive Cocci
- Staphylococcus epidermidis: (Coagulase-Negative Staphylococci)
  - Resistance: Often intrinsically resistant to penicillin and sometimes to other beta-lactams due to production of beta-lactamase. Variable resistance to aminoglycosides, macrolides, and clindamycin. High risk of resistance to multiple antibiotics
  - Clinical Significance: Frequent colonizer of skin and mucous membranes, but can cause infections associated with indwelling medical devices
- Enterococcus: species (e.g., E. faecalis, E. faecium)
  - Resistance: Intrinsic resistance to cephalosporins and low-level resistance to aminoglycosides. Variable resistance to penicillin. E. faecium is more likely to be resistant to ampicillin and vancomycin (VRE)
  - Clinical Significance: Important cause of healthcare-associated infections, including bloodstream infections, urinary tract infections, and endocarditis
- Streptococcus pneumoniae
  - Resistance: Resistance to penicillin and other beta-lactams is increasing due to alterations in PBPs. Resistance to macrolides and tetracyclines is also common
  - Clinical Significance: Common cause of community-acquired pneumonia, meningitis, and otitis media
- Streptococcus pyogenes: (Group A Streptococcus)
  - Resistance: Generally susceptible to penicillin and other beta-lactams. Resistance to macrolides is emerging
  - Clinical Significance: Causes pharyngitis, skin infections, and invasive infections
Gram-Negative Bacilli
- Escherichia coli
  - Resistance: Highly variable. Intrinsic resistance to ampicillin, amoxicillin-clavulanate, and some first-generation cephalosporins. Resistance to fluoroquinolones, trimethoprim-sulfamethoxazole, and extended-spectrum cephalosporins (ESBLs) is increasing
  - Clinical Significance: Common cause of urinary tract infections, bloodstream infections, and other infections
- Klebsiella pneumoniae
  - Resistance: Similar to E. coli, with increasing resistance to multiple antibiotics, including carbapenems (CRE) due to production of carbapenemases. Intrinsic resistance to ampicillin
  - Clinical Significance: Common cause of healthcare-associated infections, including pneumonia, bloodstream infections, and urinary tract infections
- Proteus mirabilis
  - Resistance: Intrinsic resistance to tetracycline and nitrofurantoin. Variable resistance to some other antibiotics
  - Clinical Significance: Common cause of urinary tract infections
- Pseudomonas aeruginosa
  - Resistance: Intrinsic resistance to many antibiotics, including many beta-lactams (except some antipseudomonal penicillins and cephalosporins), aminoglycosides, tetracyclines, and macrolides. Often resistant to multiple antibiotics
  - Clinical Significance: Important cause of healthcare-associated infections, including pneumonia, bloodstream infections, and wound infections
- Acinetobacter baumannii
  - Resistance: Highly resistant. Often resistant to multiple antibiotics, including carbapenems, due to production of carbapenemases. Intrinsic resistance to many antibiotics
  - Clinical Significance: Important cause of healthcare-associated infections, particularly in intensive care units
- Enterobacter: species (e.g., E. cloacae, E. aerogenes)
  - Resistance: Variable. Intrinsic resistance to ampicillin, amoxicillin-clavulanate, and some first-generation cephalosporins. Can develop resistance to extended-spectrum cephalosporins (ESBLs) and carbapenems
  - Clinical Significance: Common cause of healthcare-associated infections
Anaerobes
- Bacteroides fragilis
  - Resistance: Intrinsic resistance to aminoglycosides, trimethoprim-sulfamethoxazole, and clindamycin. Variable resistance to penicillin
  - Clinical Significance: Common cause of intra-abdominal infections, wound infections, and other anaerobic infections
Other Important Species
- Haemophilus influenzae
  - Resistance: Beta-lactamase production is common, leading to resistance to ampicillin. Resistance to chloramphenicol and trimethoprim-sulfamethoxazole is also observed
  - Clinical Significance: Causes respiratory tract infections, meningitis, and other infections
- Moraxella catarrhalis
  - Resistance: Produces beta-lactamase, leading to resistance to penicillin and ampicillin. Generally susceptible to other antibiotics
  - Clinical Significance: Causes respiratory tract infections
- Neisseria gonorrhoeae
  - Resistance: Resistance to penicillin, tetracycline, and fluoroquinolones is common. Resistance to cephalosporins is emerging
  - Clinical Significance: Causes gonorrhea, a sexually transmitted infection
- Chlamydia trachomatis
  - Resistance: No known resistance to the usual treatments
  - Clinical Significance: Causes chlamydia, a sexually transmitted infection
- Mycoplasma pneumoniae
  - Resistance: No cell wall, therefore intrinsically resistant to beta-lactams
  - Clinical Significance: Causes pneumonia

General Principles

Consult Guidelines: Always refer to current CLSI guidelines or other reputable sources for the most up-to-date information on intrinsic resistance patterns
Consider Site of Infection: Intrinsic resistance patterns can vary depending on the site of infection. For example, Enterococcus species are intrinsically resistant to many cephalosporins, but this is not always relevant in urinary tract infections
Susceptibility Testing is Essential: While understanding intrinsic resistance is helpful, susceptibility testing is still crucial to confirm the antibiotic susceptibility of each individual isolate
Report Intrinsic Resistance: Laboratories should report intrinsic resistance patterns to clinicians, especially for common species

Key Terms

Intrinsic Resistance: Natural resistance of a bacterial species to an antibiotic
Susceptibility Testing: Laboratory tests performed to determine the susceptibility of a bacterium to various antibiotics
CLSI (Clinical and Laboratory Standards Institute): An organization that provides standardized methods and interpretive criteria for antimicrobial susceptibility testing
Empiric Therapy: Treatment initiated before the causative organism is identified and susceptibility results are available
Beta-Lactamase: An enzyme that inactivates beta-lactam antibiotics
Carbapenemase: An enzyme that inactivates carbapenem antibiotics
MRSA (Methicillin-Resistant Staphylococcus aureus): Staphylococcus aureus that is resistant to methicillin and other beta-lactam antibiotics
VRE (Vancomycin-Resistant Enterococcus): Enterococcus species that is resistant to vancomycin
CRE (Carbapenem-Resistant Enterobacteriaceae): Enterobacteriaceae that is resistant to carbapenems
PBP (Penicillin-Binding Protein): Bacterial enzyme involved in cell wall synthesis
Efflux Pump: A mechanism used by bacteria to pump antibiotics out of the cell

Genetic Determinants

Antimicrobials Reporting