A 70-year-old man is admitted to the Acute Medicine Unit as he is pyrexial and feeling generally unwell. He has a history of ischaemic heart disease and had a bioprosthetic mitral valve replacement 5 years ago. An echocardiogram is arranged which shows a vegetation around the mitral valve. Blood cultures are taken which are reported as follows: Streptococcus viridans
What is the most appropriate antibiotic therapy?
A- A IV benzylpenicillin + gentamicin***
B- A IV ceftriaxone + benzylpenicillin
C- A IV flucloxacillin + gentamicin
D- A IV vancomycin + rifampicin + gentamicin
E- A IV vancomycin + benzylpenicillin.
Gentamicin is an aminoglycoside. It is used as an antibiotic to eradicate infections against sensitive bacteria.
It is used to treat various serious diseases of the skin, lung, stomach, urinary tract and blood, as well as skin and eye wounds.
Its use is indicated when the administration of other less potent antibiotics has been ineffective.
Due to its high toxicity and multiple side effects, its use should be avoided if it is not strictly necessary.
They concentrate on the ear and kidney, therefore they have ototoxic and nephrotoxic side effects.
Indications:
Some treated eye diseases are: blepharitis, conjunctivitis, dacryocystitis and keratitis.
It is also used for cleaning the connective membrane before eye surgery.
Other diseases he treats are: bacterial septicemia, peritonitis, staphylococcal pneumonia, Klebsiella pneumonia, acute otitis media, meningitis, ventriculitis, bone diseases, sinusitis and prevention of burn infections, among several others.
It is also used intratympanicly, to treat Ménière's disease in cases resistant to other treatments, as an alternative to surgery.
Gentamicin remains the aminoglycoside of choice for hospital-acquired infections due to enterobacteriaceae in institutions with minimal bacterial resistance to it.
It is common to see its association with β-lactams for these infections.
Endocarditis:
In combination with ampicillin, penicillin or vancomycin is indicated in cases of endocarditis caused by Streptococcus viridans or sensitive enterococci.
In combination with ampicillin or vancomycin, they are used in cases of patients allergic to penicillin for prophylaxis in bacterial endocarditis in high-risk patients.
In combination with anti-staphylococcal penicillins for tricuspid endocarditis caused by Staphylococcus aureus in intravenous drug users.
In association with vancomycin and rifampicin it is indicated in the treatment of endocarditis caused by negative coagulase staphylococcus in patients with valvular prostheses.
In combination with antipseudomone penicillins for the treatment of serious infections caused by Pseudomonas aeruginosa such as bacteremia, endocarditis and onset of malignant otitis externa.
In combination with ceftriaxone for the treatment of endocarditis due to penicillin-sensitive streptococcus.
Mechanism of action:
Its mechanism of action consists in interfering with the normal synthesis of proteins, originating non-functional proteins in susceptible microorganisms.
To exert their action they must enter the bacterial cell.
This occurs in 2 stages by an active transport mechanism. In the first phase, the entry into the cell depends on the transmembrane potential generated by aerobic metabolism.
The second phase is accelerated admission, and is favored by the previous binding of the aminoglycoside to the bacterial ribosome.
Certain conditions that reduce the electrical potential of the membrane such as anaerobiosis or the low pH of the medium, decrease the entry of these compounds into the bacterial cytoplasm.
Once inside the cell, the aminoglycosides bind irreversibly to the 30S subunit of the bacterial ribosome.
This union interferes with the elongation of the peptide chain.
They also cause incorrect readings of the genetic code forming abnormal proteins.
Some of these are membrane proteins and the result is the formation of channels that allow more drugs to enter the cell.
Action spectrum:
It is active against Gram negatives, especially Escherichia coli, Proteus species (indole-positive and indole-negative), Pseudomonas aeruginosa, Klebsiella species, Enterobacter, Serratia, Citrobacter, and Staphylococcus species (coagulase-positive and coagulase-negative, including strains resistant to penicillin and methicillin).
Anaerobes, such as Bacteroides or Clostridium, and most Streptococcus species are usually resistant to aminoglycosides.
