A quo­rum of flavour

If you think bac­te­ria live a low life, know­ing about their strate­gic germ warfare will change your mind – and help you un­der­stand what hap­pens to cer­tain foods at the mi­cro­scopic level.

The Star Malaysia - Star2 - - TASTE -

CER­TAIN odd things in­ter­est me – and of­ten they are things which most peo­ple would never think of, and even if they did, they would still find it bor­ing or te­dious. But it doesn’t bother me at all, and hope­fully you will soon get to see why some things are a whole lot more in­ter­est­ing than you thought pos­si­ble.

As a lover of cheeses, it is quite re­lax­ing for me to re­search how cheeses are cre­ated – and in par­tic­u­lar, the com­plex mix of bac­te­rial and fun­gal cul­tures needed to cre­ate the idio­syn­cratic char­ac­ter­is­tics and flavours of var­i­ous cheeses.

One of my favourite cheeses, St Nec­taire Fer­mier, is pro­duced by an as­ton­ish­ingly com­plex and ac­tive bac­te­rial com­mu­nity made up of de­bary­omyces hansenii, geotrichum can­didum, bre­vibac­terium linens, toru­lop­sis sphaer­ica, kluyveromyces lactis, can­dida sake, cla­dospo­rium, as­pergillus, etc. Some of th­ese same bac­te­ria are also ac­tive in the pro­duc­tion of blue cheeses as well, with the char­ac­ter­is­tic blue fur­rows de­vel­oped by ei­ther peni­cil­lium roque­forti or peni­cil­lium glau­cum.

What was im­me­di­ately cu­ri­ous is how th­ese var­i­ous mi­cro­bial cul­tures man­age to co- ex­ist so peace­fully in the cheeses. It would be more plau­si­ble to as­sume that one of the more ag­gres­sive or­gan­isms would at­tempt to wipe out the other species and dom­i­nate the whole curdy land­scape – and very likely in­tro­duc­ing a sin­gle sour taste to all cheeses.

But it seems that th­ese var­i­ous bac­te­rial and fun­gal colonies are per­fectly happy to co- ex­ist in cheeses – act­ing rather like a per­fect multi- racial hu­man so­ci­ety where each race con­trib­utes sub­tly yet sig­nif­i­cantly to the cul­tural flavour and well- be­ing of the coun­try.

Ob­vi­ously, this utopian sit­u­a­tion very rarely hap­pens, even with seem­ingly in­tel­li­gent hu­man be­ings, so how come sim­ple bac­te­ria and fungi are so much bet­ter at co- ex­is­tence?

The an­swer is rather fas­ci­nat­ing and also ex­plains why our bod­ies tol­er­ate the many bil­lions of bac­te­ria in the mi­cro­bial flora in our in­testines.

Some ev­i­dence of bac­te­rial co- op­er­a­tion was al­ready ob­served in the mid- 1960s, no­tably by a Hun­gar­ian- born sci­en­tist called Alexan­der To­masz. By 1994, sci­en­tists had de­ter­mined that bac­te­ria can in­deed de­tect vari­a­tions in their en­vi­ron­ment caused by other bac­te­ria – what’s more, the bac­terium can un­der­stand the im­pli­ca­tions of such changes and ap­ply spe­cific sur­vival strate­gies based on the en­vi­ron­men­tal al­ter­ations.

A key piece of re­search penned in 1994 by the Amer­i­can sci­en­tists, Fuqua, Wi­nans and Green­berg, sketched out the con­cept of Quo­rum Sens­ing ( QS) by bac­te­ria. The orig­i­nal re­search was based on marine lu­mi­nes­cent bac­te­ria but the prin­ci­ples of QS ( or in­ter- bac­te­rial com­mu­ni­ca­tion) has since been val­i­dated and found to ap­ply to many other mixed colonies of bac­terium, in­clud­ing those in cheeses.

At the sim­plest level, QS is man­aged by the pro­duc­tion and in­ter­play of groups of sig­nalling mol­e­cules known as au­toin­duc­ers – the in­di­vid­ual mol­e­cules are known as in­duc­ers and they are de­tected by spe­cial re­cep­tors in the bac­te­rial cells.

A sin­gle cell can emit one or more dif­fer­ent kinds of in­duc­ers. The in­ter­ac­tion be­tween in­duc­ers and re­cep­tors can be quite sim­ple or very com­plex, de­pend­ing on var­i­ous fac­tors, not least be­ing the prox­im­ity of other colonies of dif­fer­ent bac­terium in the same en­vi­ron­ment.

There are three gen­eral types of in­duc­ers – the ones pro­duced by Gram- pos­i­tive bac­te­ria are based on pep­tides ( or chains of amino acids) while Gram- neg­a­tive bac­te­ria use de­riv­a­tives of fatty acids.

If you’re cu­ri­ous, Gram- pos­i­tive bac­te­ria have thicker cell walls which can re­tain the pur­ple dye used in the Gram stain test ( hence the stain ef­fect is pos­i­tive) – so by in­fer­ence, Gram- neg­a­tive bac­te­ria are not stained by the Gram stain test and that’s be­cause their thin­ner cell walls can­not hold the stain.

The fi­nal type of in­duc­ers is rather rarer and can be pro­duced and utilised by both Gram- pos­i­tive and Gram- neg­a­tive bac­te­ria – th­ese in­duc­ers are un­usual be­cause they are based on boron, an el­e­ment sel­dom as­so­ci­ated with biomolecules.

Re­gard­less of the types of in­duc­ers, the ef­fect at the cell level is like a non- lin­ear equa­tion; that is, the ef­fect can be­come sud­denly chaotic.

How­ever, it is im­por­tant to note that au­toin­duc­ers work not only on ex­ter­nal bac­terium but also within the same colony of bac­te­ria. Within the same colony, the bac­te­ria may choose to wait and ex­pand un­til the colony reaches a cer­tain size be­fore break­ing out of their en­clave – this is how many bac­te­rial in­fec­tions be­gin.

The bac­te­ria are al­ways aware of its colony size by the amount of au­toin­duc­ers around it and won’t break out un­til it is con­fi­dent of over­whelm­ing the host’s body de­fences.

The in­ter­ac­tion be­tween dif­fer­ent species of bac­te­ria goes along some­what sim­i­lar lines. Let’s re­view a sit­u­a­tion where the num­bers of au­toin­duc­ers from an ex­ter­nal bac­terium are slowly grow­ing in an en­vi­ron­ment colonised by an­other sin­gle host species of bac­te­ria.

While the num­bers of ex­ter­nal au­toin­duc­ers are small, noth­ing much re­ally hap­pens – the in­ducer mol­e­cules are de­tected, counted by the host re­cep­tors and things con­tinue qui­etly be­tween both colonies of bac­te­ria. And it stays that way un­til the quan­tity of in­duc­ers de­tected reaches a thresh­old ( also known as the quo­rum).

Once the quo­rum is reached, the host bac­te­ria re­act rather more fre­net­i­cally and in sev­eral ways – one way is by ac­ti­vat­ing a gene which pro­duces more re­cep­tors.

An­other way is to ac­ti­vate a sep­a­rate gene which pro­motes the pro­duc­tion of its own au­toin­duc­ers – al­most like a sig­nal to alert its pres­ence to other bac­terium.

There are then sev­eral op­tions avail­able to the host bac­te­ria – they can at­tempt to de­stroy the ex­ter­nal in­vad­ing bac­te­ria, they can choose to tol­er­ate each other as peace­fully as pos­si­ble, or they can form a part­ner­ship with the ex­ter­nal bac­terium.

Within our own bod­ies, there are ex­am­ples of all three kinds of be­hav­iour. For­eign vir­u­lent bac­te­ria ( also known as pathogens), once de­tected in the blood or or­gans, are set upon by the white blood cells in a vi­o­lent at­tempt to kill the in­vaders.

How­ever, in the gut, the ex­ter­nally- in­tro­duced mi­croflora work to­gether mag­nif­i­cently with the body – in fact, much of the body’s im­mune sys­tem em­anates from, or is en­hanced sig­nif­i­cantly, by the bac­te­rial colonies there.

And within the gut bac­te­ria, there are also count­less species which don’t do much for the body but are tol­er­ated ei­ther be­cause they are food for the good bac­te­ria or they just don’t have much im­pact on any­thing and it is too dif­fi­cult to rid them from amongst the rest of the gut mi­croflora.

So al­though there are many kinds of au­toin­duc­ers swirling around in the gut, it does seem that the var­i­ous species of gut bac­te­ria gen­er­ally tend to get along fine with each other.

So from the above, you can prob­a­bly sur­mise, quite cor­rectly, that the mul­ti­ple colonies of bac­te­ria and fungi in cheeses also tol­er­ate each other pretty well. The main dif­fer­ence is that they don’t have any noble aims to ben­e­fit a higher or­gan­ism ( as with hu­man gut mi­croflora) – most of the bac­te­ria in cheeses just co- ex­ist to­gether in blocks of cur­dled milk, pro­duc­ing their in­di­vid­ual mi­cro­bial end prod­ucts, and the fi­nal re­sults are the idio­syn­cratic flavours and tex­tures of cheeses.

As an aside, if you have been drink­ing al­co­hol, the ethanol can sig­nif­i­cantly im­pact the mi­croflora in the gut, mainly by killing them. Hence it is al­ways a good idea to in­gest some pro­bi­otic prod­ucts the next day to re­place the lost bac­terium. The al­ter­na­tive, of course, is to avoid drink­ing al­co­hol but that would be as ridicu­lous as the idea of me con­vert­ing to a re­li­gion.

hos­pi­tal- as­so­ci­ated Me­thi­cillin re­sis­tant Sta­phy­lo­coc­cus au­reus ( MrSA) bac­te­ria. — Pho­tos:


Probiotics, the nat­u­ral friendly bac­te­ria are an in­te­gral part of the di­ges­tive sys­tem.

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