MICROBIAL INFECTIONS OF HUMANS(HUMAN MICROBIOLOGY CONTD..)
The ability to reproduce will be the ultimate driving force for all organisms. For bacteria that infect
humans there are several routes that can be employed to infect new hosts. The various means by which a
micro-organism may be transmitted from the source to the new host.Note that some organisms can be spread by more than one mode of transmission. Measles virus is likely to be spread by airborne and direct
contact. The number of times people touch their faces with their hands is enough to almost guarantee
that virus in the nose or saliva will be transferred to the hands. You will notice other flaws in the
scheme. The table distinguishes between sexually transmitted infections and those obtained by direct
contact. It could be argued that they are the same mode of transmission. The example of dermatophytes
as directly transferred infections also needs qualifying. In the latter cases the fungi are often transmitted
through shared, inanimate objects like towels or combs. This is not direct transmission in the strictest
sense. Perhaps ‘indirect transmission’ is a more appropriate term.
Two important terms are vertical transmission and horizontal transmission. Rather than referring to
positions of the host when they came into contact with the infectious agent (!) they are used to
distinguish between infections transferred from mother to child (vertical) and those acquired from other
sources (which in this context means predominantly people) and are called horizontal. Two of the most
important examples of vertical transmission are rubella virus and Treponema pallidum(syphilis). Such
infections of the foetus can have serious developmental consequences. Congenital rubella, for example,
may lead to microcephaly (small head), impaired vision and impaired intellectual development.
The significance of vertical or horizontal transmission for the microbe lies in the number of people that
can be infected. With vertical transmission only one new host is infected and that is the child (or
children). With horizontal transmission there is the opportunity to infect significantly greater numbers of
people. It follows then that if one person is able to infect five others, these five will then infect 25 and so
The exact point at which infections of the newborn baby stop being vertical transmission and become
horizontal is debatable. Certainly vertical transmission can be subdivided into:
• germline transmission (spread of virus integrated within the genome),
• prenatal transmission (infection of the foetus in the uterus),
• perinatal transmission (infection during birth),
• postnatal transmission (infection following birth).
Strictly, only infections of the foetus in utero are vertically transmitted. Perinatal infections (those
acquired by the baby in the vaginal tract during the process of delivery) and postnatal infections (e.g.
acquired via breast milk) are horizontally transmitted.
Vector-borne infections are those that are transferred by a vehicle/carrier, which are insects. The term
vector is (most usefully) restricted to living animals such as lice, fleas or mosquitoes. Combs and
hairbrushes are thus not usually called vectors, but fomites. The transfer of dermatophytes via
hairbrushes is best described as ‘direct transfer’. Vectors serve to transmit organisms from the
bloodstream of infected patients to new susceptibles. The organisms (notably arboviruses: arthropod borne viruses and protozoan parasites such as malaria) are not designed to withstand any time outside of
a host. The transfer of the organism is dependent on the distances the vector can travel in its lifetime. A
wide range of insects transmit infectious microbes.