Fibrils, Sex and Fuzzy Coats
Limitation of Light
One of the frustrating
aspects of working with bacteria is that they are so small that it is almost
impossible to see anything other than their shape when looking down even the
very best of optical microscopes. Even then, their refractive index is so
similar to that of water that they have to be stuck to a glass slide, killed and
stained before even their shape is revealed. Microscopes which can make use of
polarized light (Phase contrast microscopy) can be used to see living bacteria
but apart from the added ability of seeing some species happily swimming around
they add little to what we can see using conventional staining techniques.
The fact that some
species could move quite rapidly intrigued many early microbiologists and
eventually some special staining procedures lead to the discovery of thin
whip-like appendages which they called flagellae and conferred motility.
This is not to say that light microscopy is not useful. It remains an
essential tool in any bacteriology laboratory but it should be recognized that
the information obtained, although extremely helpful in routine work, is
Microscopy Reveals More
The invention of the
electron microscope revealed much more detail of bacteria. Compared to the
fascinating structures uncovered in eukaryotic cells, bacteria, both inside and
out, were pretty uninteresting. It
wasn't until the early 1960s that some interesting surface features of some
bacterial species were noticed. This delay was partly due to the electron
microscopy techniques in use at that time. The convention at the time was to use
ultra-thin sections of tissue, far thinner than sections used for light
microscopy. It seemed normal then to prepare bacteria in the same way. Using
these techniques, the outer surfaces of bacteria seemed fairly barren but the
technique did reveal some of the double membrane-like composition of
Reveals Still More
Although thin sections of bacteria did not allow flagella to be seen in their entirety it did reveal interesting cross-sections which showed their internal structure. It also enabled detail of flagella attachment to be demonstrated. It was not until electron microscopes were used to look at whole cells rather than ultra-thin sections that more progress was made. This change required the development of new staining techniques known as shadow-casting where bacterial surfaces were sprayed with electron-dense material such as gold or carbon at an angle. This highlighted the fine surface structures in a way exactly analogous to light falling on a stone surface at an angle reveals more detail than light falling on it at right angles.
Flagellae and Fimbriae
techniques had been developed the whip-like flagellae were the first to be
examined in detail but one researcher in particular noticed the presence of
previously undreamed of structures on the surface of some species.
The person who first described these structures which he found on strains
of Escherichia coli and Salmonella was Professor James Duguid. He
called them fimbriae.
What are Fimbriae?
are thin, hair-like, projections made of protein sub-units. A number of
different types have been described (about 7 at the last count, labeled Types
I-VII) which can be distinguished by their size (length and diameter) and the
type of antigens they carry. They
are characteristic of some Gram-negative bacteria such as Escherichia coli
and Salmonella spp and were first described back in the 1960s by JP
Duguid who was the Professor of Microbiology at the
A short while
after Duguid published his findings an American called Robert Brinton
published much the same stuff and called them pili. What followed was a
pretty acrimonious exchange of letters in the scientific press about
what they should be called.
It was all pretty
good fun but to this day our American cousins, and anybody who doesn't
know any better, call them pili whereas all right-thinking, clear-minded
and fair microbiologists refer to them as fimbriae.
So What do
Fimbriae Actually do?
Over the years we have
learned quite a lot about fimbriae and right from the very early days it was
thought that they were involved in helping the bacteria adhere to surfaces.
There is now a substantial body of evidence in support of this much of it in
relation to pathogenic strains of E coli.
Type I Fimbriae are Pathogenicity Factors
It's clear these days
that Type I fimbriae are involved in bacterial adhesion and the very best
example are those carried by pathogenic strains of E.coli. These come in
a variety of forms including plain old EnteroPathogenic E.coli (EPEC),
EnteroToxigenic E.coli (ETEC), EnteroInvasive E.coli (EIEC) and VeroToxogenic
E.coli (VTEC). These E.coli strains use Type I fimbriae to adhere to gut
mucosal cells which is the first step in the pathogenic process. Without the
fimbriae their capacity to cause disease is greatly diminished or abolished
Type IV fimbriae are
particularly interesting. These have also been referred to as "bundle
forming pili" because of their ability to aggregate into bundles. These
fimbriae are thought to be connected with the ability of EPEC strains to form
microcolonies on tissue monolayers and mutants lacking this ability show reduced
virulence. Type IV fimbriae have also been shown to be involved in the
remarkable phenomenon of bacterial twitching motility which allows bacterial
cells to crawl over a surface.
Type VII Fimbriae,
Viruses and the Sex Bit
Type VII fimbriae are the
conduit for DNA transfer between bacterial mating strains. As it happens they
also provide a binding site for certain bacteriophages. The significance of this
is a mystery but it does enable Type VII to be seen clearly because when some of
the bacteriophage is added to a suspension of cells, the 'phage coat the Type
VII fimbriae. In the electron
microscope picture above right you can clearly see little particles stuck on two
of the fimbriae which are much longer than the rest because size does matter, at
least to E.coli. In a generous attempt to resolve the fimbriae/pili
argument it was proposed that Type VII fimbriae were named the "sex
Surfaces of Streptococci
Back in the days before we knew much about fimbriae researchers looking at ultra-thin sections of the serious pathogen Streptococcus pyogenes noticed that the very outside of the cells had a fuzzy appearance. In a fit of imagination it was called "fuzzy coat". Later, when they learned about shadow-casting whole cells they applied this technique but it did not help to resolve any particular structures like fimbriae.
S. pyogenes Fuzzy Coat
Even today we have not resolved any definite structure to the S. pyogenes "fuzzy-coat". We do know, however, that it consists partly of a substance called "M-protein" which is a major pathogenicity factor of this species.
Reveals Surface Fibrils on Some Streptococci
Towards the late 1970s a
rather different technique which made use of a special type of stain called a
"negative stain" revealed very thin, delicate, hair-like structures on
some oral streptococci such as Streptococcus sanguis and Streptococcus
salivarius. Take a look at the photograph on the right. This is an electron
micrograph of the surface of a Streptococcus salivarius cell and although
it may not be terribly clear on this reproduction, the original shots showed two
types of these thin hair-like structures, long ones and short ones.
This negative-staining technique could not, by the way, reveal anything
hair-like on the surface of Streptococcus pyogenes which had the fuzzy
Fibrils are not
More research using lots
of different strains of different species of oral streptococci showed these
"hairs" came in all sorts of lengths and some cells carried more than
one type. They were very thin and flexible. Although some fimbriae on E.coli
can be very thin, "flexible" is not a term normally associated with
fimbriae. To begin with these hairs
were called "fibrils" and there is a fair amount of evidence to
suggest they are made of protein and some evidence which suggests that some are
even made of glycoprotein although glycoproteins are generally considered pretty
rare beasts in bacteria. As far as fibril synthesis goes, we don't know much.
Generally speaking they are difficult to remove, probably because they are so
flexible, so it's not possible to say whether they can re-grow like fimbriae.
The analogy was taken a stage further when a role in adhesion was
postulated and, in fact, there is fairly good evidence to back this up, at least
for the S.salivarius fibrils.
Unfortunately at this
point the waters got a bit muddy when some people started referring to the long
fibrils as "fimbriae" and the short ones as "fibrils". Since
they are kind of like fimbriae this wasn't so surprising but what was surprising
was that they were never referred to as pili!
Oral Streptococci Have Tufts of Fibrils
strains of oral streptococci were found to carry tufts of fibrils and
looked rather like punk-rockers with Mohican hairstyles. Later these
were grouped together into a new species and given the rather elegant
name Streptococcus cristae.
There is evidence that these may also be involved in adhesion, this time to rod-shaped bacteria to make the structures commonly found in mature dental plaque called "corn-cob-configuration". When bacteria of the same species stick to each other it's known as "aggregation". In this case the bacteria are from different species and it's known as "CO-aggregation".
"Corn Cobs" in Dental Plaque
have guessed by now that I'm a bit skeptical about using the term
"fimbriae" to describe the surface structures of these oral
streptococci. I prefer to describe them all as fibrils but I'll probably end up
in the minority. Sooner or later
this is all going to be resolved but for the time being it's probably best to
keep the term "fimbriae" reserved for those brittle hair-like,
proteinaceous surface projections of Gram-negative rods like Escherichia and
Salmonella and call everything else "fibrils".
Just remember pili are
fimbriae and fibrils are different and you won't go far wrong.