Question

Consider the factors that impart pathogenicity in some staphylococci.

How do these factors operate in the host in relation to the disease process?

Discuss:

• the mode of action on the bacterial cell of each antibiotic tested
• the mechanisms by which bacteria exert resistance to these antibiotics and
• the concepts of action of bactericidal and bacteriostatic antibiotics

• What do you think are important factors that relate to the patient, the pathogen, the administrator and the community that need to be considered when deciding on an antimicrobial drug of choice for therapeutic use?

How should the antibiotic be administered?

Answer 1

• Factors imparting pathogenicity to some Staphylococci:

1. Protein A: It is a surface protein which binds to the Fc region of host immunoglobulins rendering them inactive for opsonization and complement pathway activation. It thus takes a toll on humoral immunity as antibody action fails to eradicate this pathogen making the host equally prone to repetitive Staphylococcal attacks. Staphylococcal Pneumonia is caused due to the ability of Protein A to bind to Tumour Necrosis Factor 1 (TNF 1) of lung tissue which leads to lung inflammation. Protein A has been found to be one of the factors involved in promoting biofilm formation on catheters and other medical implants inside the host.
2. Bound coagulase or clumping factor: Coagulase is like prothrombin, which helps convert fibrinogen to fibrin and initiates blood clotting. This is an exclusive pathogenic trait of Staphylococcus aureus.
3. Techoic acid: This is a membrane bound moiety which helps Staphylococci bind to mucosal membranes and infect them. It has been shown to act similar to endotoxin and prove as a inducer of septic shock in the host body (a condition characterized by hypotension in which organs fails to receive oxygen due to circulatory defect). It has been shown to cause degranulation of mast cells by binding to neutrophils, causing hypotension.
4. Superantigens: Enterotoxins A-D are heat and acid resistant proteins which enter the intestine by resisting the stomach acidity. In the intestine, they are absorbed by the intestinal mucosa and help generate a hyperactive immune response which leads to localized inflammation and diarrhoea. Toxic Shock Syndrome Toxin – 1 (TSST-1) superantigen hyperstimulates the immune system into releasing large amounts of interleukins, Tumour Necrosis Factor (TNF) and cytokines resulting in the fatal Toxic Shock Syndrome (TSS).
5. Haemolysins: alpha- haemolysin produced by S. aureus, attacks platelets and monocytes by binding to their surface and drilling their contents out. Leukocidin is another toxin which is less aggressive than alpha- haemolysin but plays an important role in necrotizing skin infections.
6. Enzymes: Enzymes like Coagulase, Peroxidase, Catalase etc. help in paralyzing the immune system by making unavailable the key components required for their action. For ex: Catalase degrades H₂O₂ thereby hampering the action of neutrophils (the first immune cells to reach the site of infection).

• Mode of action of antibiotics:
  1. Beta-lactams: These antibiotics include Penicillin, Methicillin, Vancomycin, Cephalosporins etc. and possess a beta-lactam ring structure. They act on the pathogen by inhibiting cell wall synthesis. They inhibit the final transpeptidation reaction, a reaction in which cross-linking of glycans takes place. They bind to Transpeptidase (Penicillin Binding Protein-pbp) thereby inhibiting its activity.
  2. Translation inhibitors: Clindamycin is the most common antibiotic used which inhibits ribosomal translocation onto the messenger RNA, thereby inhibiting protein synthesis.
  3. Lincomycin is a protein synthesis inhibitor drug which binds to the 50S ribosomal sub-unit and prevent peptide bond formation during translation.
  4. Tetracyclin interferes with the tRNA anticodon reading of the mRNA codon.
  5. Erythromycin binds to the 23S rRNA and inhibits exit of peptide from the ribosome thereby blocking protein synthesis.
  6. Rifampin binds to RNA polymerase, forming a drug-enzyme complex, rendering it inactive for transcription.
• Drug resistance:
  1. Beta-lactam resistance: This can occur via 3 mechanisms:
     • Production of Beta-lactamase, an enzyme that degrades the beta-lactam ring of the antibiotics, rendering them inactive.
     • Evolution of beta-lactam insensitive transpeptidase enzymes which do not bind to beta-lactams, thereby continuing cell wall formation.
     • Excessive pumping out of beta-lactams from inside the bacterial cell. This occurs by evolution of pumping mechanisms on the cell surface.
  2. Resistance to drugs binding to ribosomal RNAs majorly includes a change in structure in the ribosome-drug binding site. The organisms have also been reported to have developed pumping mechanisms to increase the efflux of macrolides and other translational inhibitors.
  3. Development of RNA polymerases insensitive to the action of drugs like rifampin which inhibit them have been found to have evolved in resistant Staphylococci.
• Bactericidal and Bacteriostatic antibiotics:

Bacteriostatic antibiotics are reversible antibiotics which temporarily inhibit the growth of the pathogen. Their effect is not permanent in the sense that the organism will grow if they are removed from the treatment. For instance, if a transcriptional inhibitor binds to RNA polymerase and doesn’t in turn allow it to bind to the template DNA, in the antibiotic’s absence, the RNA polymerase would be free to bind to the template and resume transcription.

Bactericidal antibiotics on the other hand kill 99% of the pathogen thereby eliminating them completely. Withdrawing them from treatment will not enable regrowth of the damaged pathogens in the host.

• Important factors to be considered for drug selection:
  1. Type of infection: Not all pathogenic Staphylococci affect all the parts of the host. Some infect the lungs, some infect the intestine, some infect the skin etc. Thus the type of infection will majorly decide the causative species of Staphylococci, thereby enabling us to determine an appropriate drug.
  2. Lab diagnosis: In case of severe infections, a complete microbiological analysis of the pathogenic strain has to be carried out in order to determine the spectrum of antibiotics the particular strain is sensitive to.
  3. Geographical location: Distribution of sensitive and resistant strains is dependent on the geographical location since resistance develops after prolonged exposure of the antibiotic. If a location has had multiple outbreaks of the said infection, chances of prevalence of resistant strains are higher.
  4. Patient history: The administrator has to take in to consideration the history of the patient i.e. the type of antibiotics he/she is allergic to (if any).
  5. Dosage: The dosage of the antibiotic should be appropriate enough to ensure complete eradication of the pathogen from the host system.
  6. Combination or singular therapy: In case of suspected resistance, a combination therapy should be used which includes drugs with different modes of action. This will ensure that resistance to one particular drug will not hamper the eradication of the organism from the system.
  7. Completion of the dose: The patient should ensure that he/she completes the dosage for the prescribed duration. Symptoms may vanish but that doesn’t ensure eradication of pathogen from the system and stopping ingestion of drugs at this point may result in their regrowth leading to re-infection.
  8. Community: Staphylococcal Diseases are contagious and hence, there should be a good level of hygiene maintained around the patient. Staphylococcal infections are generally nosocomial in nature and thus, cleanliness of the hospitals must be a priority. There must be sections which separate wards containing immuno-compromised patients (burn ward or post operational wards) from the general wards.
• Antibiotic administration would depend upon the severity of the infection. In case of burn ward or immuno-compromised patients, where the effect of drugs needs to be as quick as possible, an Intravenous or Intramuscular drug administration is suggested. In case of skin infections, topical ointments can be used. In case of biofilm formation on medical implants, either the implant should be made such that it dissipates antibiotics as and when necessary or oral administration is recommended. In case of urine or less severe infections or infections in initial stages, an oral route of administration is generally the first choice.