trans

424 DRUG RESISTANCE
million new infections annually. The consequences
of infections can range from asymptomatic
to an acute inflammatory disease. The
parasite is an anaerobic protozoan flagellate
which lacks mitochondria and peroxisomes but
has a specialized double-membrane-bounded
organelle called the hydrogenosome. The
hydrogenososme is critically involved in metabolic
processes that extend glycolysis. An
important metabolic function of the hydrogenosome
is in oxidative decarboxylation of
pyruvate to acetyl CoA via a ferredoxin-mediated
electron transport system. The key
enzyme involved is the iron–sulfur-containing
pyruvate ferredoxin oxidoreductase (PFOR),
and the transport of electrons generated by
PFOR requires [2Fe–2S] ferredoxin and [Fe]
hydrogenase. The drug of choice against
Trichomonas vaginalis is the 5-nitroimidazole
metronidazole (Figure 16.5J).
Metronidazole resistance
Metronidazole enters the parasite and the
hydrogenosome by diffusion. Within this key
organelle metronidazole competes with hydrogenase
for the electrons produced from the
PFOR system. This results in the one-electron
reduction of the nitro group within the drug.
The consequent production of highly reactive
nitroso species is considered to be central to
the mechanism of action of this drug.
Metronidazole resistance has been identified
in field parasite isolates and in parasite lines
selected under drug pressure both in vitro and
in vivo. There is evidence for altered hydrogenosome
morphology in resistant parasites, the
relevance of which is not clear. Two types of
resistance have been characterized biochemically,
depending on the conditions required
to demonstrate altered susceptibility. These
are an anaerobic mechanism and an aerobic
mechanism. The anaerobic mechanism of
metronidazole resistance is associated with
a loss or elimination of the PFOR activating
pathway, notably a loss of PFOR activity. As
metronidazole resistance is increased, ferredoxin,
hydrogenosomal malic enzyme and
NAD: ferredoxin reductase levels decrease.
Loss of this key pathway is associated with a
progressive increase in 2-oxoacid oxidoreductases
as a compensatory mechanism incapable
of activating metronidazole. There remains
controversy as to whether this resistance mechanism,
readily induced in vitro, operates in field
isolates.
Aerobic resistance is only seen in the presence
of oxygen and is associated with impaired
oxygen scavenging. It is argued that the
increased intracellular oxygen concentrations
are involved in the re-oxidation of nitro free
radicals or the competitive removal of electrons,
thereby interfering with drug activation.
Importantly, most, but not all, strains responsible
for clinical resistance display aerobic
resistance. These organisms remain susceptible
to metronidazole under anaerobic conditions.
There is evidence that, during the acquisition
of anaerobic resistance, parasite isolates first
develop the aerobic resistance mechanism
which was detectable in the presence of
oxygen but not under anaerobic conditions.
PFOR activity in these isolates was significantly
lower than that in the parent strain. In
addition to these mechanisms, there is evidence
to support a role for reduced levels or
altered redox characteristics of ferredoxin and
altered hydrogenase activity in some drugresistant
trichomonads. Several clinical isolates
have been characterized as having
reduced ferredoxin levels due to a reduction
in transcription associated with alterations
in upstream regulatory regions of the gene.
An alternative resistance mechanism based
MEDICAL APPLICATIONS