Generally, when people (either general public or a physician or laboratory personnel) listen/read the term “antibiotic resistance” they think that, a bacterium which was previously sensitive to a particular antibacterial agent has now developed resistance against it (either through acquisition of gene via horizontal gene transfer or by mutation) but in this universe there are innumerable species of bacteria which are innately resistant to a particular drugs.
According to CLSI, “Intrinsic resistance is so common that susceptibility testing is unnecessary. For example, Citrobacter species are intrinsically resistant to ampicillin”.
Intrinsic antibiotic resistance is a naturally occurring phenomenon which is independent of previous antibiotic exposure and is not caused by a horizontal gene transfer. Remember the famous example of intrinsic resistance, penicillin not working against Mycoplasma. Penicillin kills bacteria by interfering with their cell wall synthesis.
Will it be able to kill those that do not have cell wall?
Intrinsic antibiotic resistance is mainly mediated by impermeability of cellular envelopes, the activity of multidrug efflux pumps or lack of drug targets. Enzymes (such as transferases) which are involved in basic bacterial metabolic processes also confer intrinsic resistance in some bacterial species such as Pseudomonas aeruginosa, and Staphylococcus aureus.
According to the published findings such natural insensitivity can be due to:
- lack of affinity of the drug for the bacterial target
- inaccessibility of the drug into the bacterial cell
- extrusion of the drug by chromosomally encoded active exporters
- innate production of enzymes that inactivate the drug
Intrinsic Antibiotic Resistance in Gram Negative Bacteria
Bacterial pathogen that are Intrinsically resistance to Ampicillin
- Acinetobacter baumanni complex
- Citrobacter freundii
- Citrobacter koseri
- Enterobacter aerogenes
- Enterobacter cloacae complex
- Klebsiella pneumoniae
- Morganella morganii
- Proteus vulgaris
- Pseudomonas aeruginosa
- Serratia marcescens
- Yersinia enterocolitica
Bacterial pathogens that are Intrinsically Resistant to Amoxicillin-Clavulanate combination
- Citrobacter freundii
- Enterobacter aerogenes
- Enterobacter cloacae complex
- Morganella morganii
- Pseudomonas aeruginosa
- Serratia marcescens
- Yersinia enterocolitica
Bacterial pathogens that are Intrinsically Resistant to Ampicillin-sulbactam combination
- Acinetobacter baumanni complex
- Citrobacter freundii
- Citrobacter koseri
- Enterobacter aerogenes
- Enterobacter cloacae complex
- Proteus vulgaris
- Pseudomonas aeruginosa
- Serratia marcescens
Citrobacter koseri is intrinsically resistant to Piperacillin whereas Proteus spp is intrinsically resistant to Tetracycline/Tigecycline, Nitrofurantoin and Polymyxin B Colistin
Acinetobacter baumanni complex is notorious pathogen which is resistant to most of the available antibiotics. It is intrinsically resistant to
- Ampicillin, Amoxicillin
- Ampicillin-sulbactam
- Aztreonam
- Ertapenem
- Trimethoprim
- Chloramphenicol
Similarly Pseudomonas aeruginosa is intrinsically resistant to
- Ampicillin, Amoxicillin
- Ampicillin-sulbactam
- Amoxicillin-Clavulanate
- Cefotaxime
- Ceftriaxone
- Ertapenem
- Tetracyclines/Tigecyclines
- Trimethoprim
- Trimethoprim-sulfamethoxazole
- Chloramphenicol
Both Acinetobacter and Pseudomonas are also intrinsically resistant to penicillin (ie, benzylpenicillin), cephalosporin I (cephalothin,cefazolin), cephalosporin II (cefuroxime), cephamycins (cefoxitin, cefotetan), clindamycin, daptomycin, fusidic acid, glycopeptides(vancomycin, teicoplanin), linezolid, macrolides (erythromycin, azithromycin, clarithromycin), quinupristin-dalfopristin, and rifampin.
Bacteroides spp. which is one of the most frequently isolated anaerobic Gram negative bacilli is intrinsically resistant to
- Aminoglycosides
- Penicillin and
- Ampcillin
Intrinsic Antibiotic Resistance in Gram Positive Bacteria
Among gram positive bacteria, S. saprophyticus is intrinscially resistance to Novobiocin which is the basis for Novobiocin sensitivity test done in urine isolate (if CONS is isolated).
Enterococcus faecalis/faecium are intrinsically resistant to
- Cephalosporin*
- Aminoglycosides*
- Clindamycin*
- Trimethoprim
- Trimethoprim-sulfamethoxazole*
- Fusidic acid
*may appear active in vitro but are not effective clinically and should not be reported as susceptible.
Both Enterococci and Staphylococci are also intrinsically resistant to aztreonam, polymyxin B/colistin, and nalidixic acid.
Anaerobic Gram positive bacilli, Clostridium spp. is resistant to Aminoglycosides.
References and further reading
Hello Sir,
I am very intersting with that communication. I am now working and Learning microbiology at the University of Kinshasa. I would be happy if you can send me supplies in mediums and usuals techniques.
I really read your article and want to say that in Kinshasa also we have resistance due to bad use of antibiotic. And also,many patients don’t do antibiogram because it cost a lot and many lab can’t do it.
Dr EKILA IFINJI Richard
Junior Assistant
Microbiology
Cliniques Universitaires de Kinshasa
[email protected]
Dear Dr. Eklia Ifinji Richard
Thank you so much for your compliment and sharing your knowledge. Increase in antimicrobial resistance is now a global problem, we are also going through similar problems here. I have listed commonly used tests in this title: http://microbeonline.com/bacterial-identification-methods/. Please go through it and leave me comments. Keep in touch.