Editor's Note: Antimicrobial resistance has become an unavoidable global public health issue. It is estimated that by 2050, the worldwide losses caused by antibiotic resistance could reach $100 trillion. In response to the challenges posed by microbial resistance, 13 ministries, including the National Health Commission of the People’s Republic of China, jointly issued the National Action Plan to Contain Antimicrobial Resistance (2022-2025) in 2022. Clinicians must rationally select antibiotics based on susceptibility reports and the patient’s condition to not only cure infections effectively but also curb bacterial resistance. Recently, Dr. Li Gu from the Department of Infectious Diseases and Clinical Microbiology at Beijing Chao-Yang Hospital, Capital Medical University, shared insights into the clinical interpretation and application of antimicrobial susceptibility reports.Bacterial Resistance Determination
There are many types of pathogenic bacteria, each with different inherent and acquired resistance mechanisms, and their susceptibility to various drugs changes dynamically. Additionally, the chemical structure and action mechanisms of antimicrobial agents are complex, necessitating in vitro susceptibility testing to assess the sensitivity of clinical isolates to different antimicrobial agents. This helps predict the clinical efficacy of antimicrobial drugs, guiding clinicians in drug selection and implementing personalized treatment. Understanding these reports is essential for clinicians.
Dr. Li Gu analyzed the key points of interpreting and applying antimicrobial susceptibility reports through nine clinical cases.
Recommendations for CRE Phenotype and Resistance Gene Detection
Regarding the necessity of testing the carbapenemase phenotype and resistance genes in carbapenem-resistant Enterobacteriaceae (CRE) isolates, the Guidelines for the Diagnosis, Treatment, and Prevention of Carbapenem-Resistant Gram-Negative Bacterial Infections (2023) suggest that since the majority of CRE strains produce carbapenemase, testing the phenotype or genotype of carbapenemase has great value in predicting resistance to carbapenems. Moreover, carbapenemase gene detection is rapid and accurate, providing early guidance for the selection of antimicrobials. If susceptibility results for newer β-lactamase inhibitor combinations (such as ceftazidime-avibactam) are not available, it is recommended that capable medical institutions perform carbapenemase phenotype or gene testing for CRE. For institutions without the ability to test for resistance genes, carbapenemase phenotype detection is suggested.
Gram-Negative Bacteria
For Gram-negative bacteria, microbiology laboratories should consider testing and reporting appropriate antimicrobial agents. The Chinese Expert Consensus on Laboratory Diagnosis, Antimicrobial Therapy, and Hospital Infection Control of Extensively Drug-Resistant Gram-Negative Bacterial Infections recommends specific antibiotics for various Gram-negative bacteria: 24 for Enterobacteriaceae, 13 for Pseudomonas aeruginosa, 17 for Acinetobacter baumannii, and seven for Stenotrophomonas maltophilia.
Carbapenem-Resistant Gram-Negative Bacteria
For infections caused by carbapenem-resistant Gram-negative bacteria (CRGNB), the Guidelines for the Diagnosis, Treatment, and Prevention of Carbapenem-Resistant Gram-Negative Bacterial Infections (2023) recommend testing the minimum inhibitory concentration (MIC) of commonly used antibiotics such as carbapenems, ceftazidime-avibactam, ceftolozane-tazobactam, imipenem-cilastatin-relebactam, meropenem-vaborbactam, cefiderocol, tigecycline, eravacycline, polymyxins, and fosfomycin.
Methicillin-Resistant Staphylococcus aureus Resistance Issues
The resistance of Staphylococcus aureus has become increasingly severe. Historically, penicillin was an effective treatment for infections caused by this bacterium. However, most strains now produce β-lactamase, resulting in widespread community resistance to penicillin. Complicating matters, methicillin-resistant Staphylococcus aureus (MRSA) is not only resistant to all β-lactams but some strains also show resistance to other classes of antimicrobials. Hospital-acquired MRSA (HA-MRSA) can cause severe outbreaks in healthcare settings, while community-acquired MRSA (CA-MRSA) typically appears in isolated cases but poses a growing public health challenge.
Treatment of MSSA Infections
For methicillin-susceptible Staphylococcus aureus (MSSA) infections, it is not recommended to empirically use broad-spectrum β-lactams or β-lactamase inhibitor combinations. Evidence shows that initiating treatment with β-lactamase-resistant penicillins or cefazolin is more effective. Studies indicate that outcomes are poorer when treating MSSA bloodstream infections with vancomycin compared to cefazolin or β-lactamase-resistant penicillins. In patients with severe β-lactam allergies, vancomycin, daptomycin, or linezolid can be considered as alternative therapies.
MRSA Treatment Options
When treating MRSA infections, the selection of therapy must consider several factors. First, it is essential to distinguish between hospital-acquired (HA-MRSA) and community-acquired (CA-MRSA) infections, as they differ in resistance patterns and epidemiology. Second, the infection site—whether in the lungs, skin and soft tissues, bloodstream, or bones and joints—directly influences treatment strategies, as different drugs are recommended for various sites of infection. The severity of the illness, patient age, comorbid conditions, and liver and kidney function are crucial in determining the treatment plan, as these factors can affect drug metabolism, excretion, and toxicity, requiring individualized treatment adjustments.
Streptococcus pneumoniae Pneumonia
When discussing the treatment of Streptococcus pneumoniae pneumonia, it is important to note that breakpoint determinations for cerebrospinal fluid and respiratory tract specimens differ, necessitating different interpretations of the report based on specimen type. If the S. pneumoniae strain isolated from respiratory specimens is penicillin-sensitive, it can be reasonably assumed that the strain is also sensitive to a range of β-lactam antibiotics, including ampicillin (oral or injectable forms), ampicillin-sulbactam, amoxicillin, amoxicillin-clavulanate, and various cephalosporins such as cefaclor, cefdinir, cefditoren, cefotaxime, cefpodoxime, cefprozil, ceftaroline, cefuroxime, and others.
For S. pneumoniae strains isolated from cerebrospinal fluid, if penicillin resistance is detected, MIC testing for cefotaxime, ceftriaxone, or meropenem should be routinely performed. Additionally, MIC or disk diffusion methods can be used to test vancomycin sensitivity for these isolates. Rifampin should not be used as monotherapy for meningitis treatment. E-test strips can be used to determine ceftriaxone MIC in hospitals, and if ceftriaxone MIC is unavailable, penicillin MIC can be referenced according to specific recommendations in the Fever guideline.
Summary
National policies focusing on curbing antimicrobial resistance have prompted clinicians to pay greater attention to the rational use of antibiotics, aiming to cure patients while effectively containing the development of bacterial resistance. In this context, accurate interpretation of in vitro susceptibility results is critical, requiring comprehensive analysis based on specimen type, bacterial identification, and specific resistance phenotypes. Microbiology laboratories provide detailed MIC susceptibility reports, which are key references for empirical or targeted treatment. Importantly, any interpretation of microbiological test reports must be grounded in clinical reality, taking into account the severity of the patient’s condition, the infection site, and individual factors such as liver and kidney function to determine the most appropriate treatment plan.
