This is an idea I've been toying with about a month now, inspired from the few odd episodes of Witch Hunter Robin I've caught on TV. The whole concept is still in the raw so it isn't completely fleshed out yet and I'm open to suggestions and changes.

Fluorescing Astral Bacteria Strain IV
This newest mutation of FAB that researchers have dubbed FAB IV, is an odd form of symbiotic/parasitic bacteria, most often found as a part of a Strain III cloud.
Like Strain III, FAB IV is a form of mana feeder, it works in symbiosis with strain III feeding from the clouds magic/essence drain and in return it provides a form of spell defense making it more difficult to kill the cloud via the usual methods (Stereilize, Cure Disease, Manaball spells).
Research into the the properties of this new strain have provided amazing results, cheif of which has been the discovery that when isolated and cultured under the right conditions Strain IV can provide any living creature, Awakened of mundane, with a form of spell defense.

FAB IV when isolated, is put into a small vial or tube of nutrient suspention and can be worn. FAB IV will remain dormant until it comes into contact with a living creature's aura, at which point it will become astrally active and bond itself to it's wearers astral aura, providing Spell Defense dice equal to it's force. The bonding changes the bacteria's astral signature to synch with it's wearer (+2 TN to astrally ID the FAB from it's linked wearer) and it begins to drain mana through hosts aura. FAB IV does Temporary Essence Drain (or Magic drain if used by an awakened char) as per the critter power, but uses the FAB III drain table (pg 91 MitS). Magically active characters who use FAB IV in addition to the temporary Magic loss also suffer a +2 target number to all magic and drain rolls due to the constant mana sucking of the bacteria.

When compined with a Stain III cloud, Strain IV provides spell defense equal to it's force to the cloud and the two clouds add their combined Force for calculation of the Draining Table Force (MitS Pg 91).

Interesting, free spell defence for mundanes. I don't see mages using it lots, due to the ill effects, but as far as adding to the bullet-proofness to street sams....(not trashing ithe idea) i don't have MitS, but how long before an average (essence 1) sam feels ill effects of the essence drain? but defiinitly a cool idea, would make mage containment much easier.

How much spell defense, how long does it last, and lastly, what's the price/availability/street index?

Given that this is a newly discovered/developed strain they shouldn't be available on the street at all. But might be the target of a run.

Personally I find all the astral bacteria a bit scary, I don't want any of them. I'll just stay away, awakened or not, that last strain kill you over time, even if they protect you from spells while doing so.
What is their effect on spirit attacks.

Awesome idea. Very creative. I may even use it in may game. Does the way you envision FABIV only protect against harmful spells or would it also make beneficial spells like Heal and increased reflexes more diffucult?

It is not made clear whether or not your idea involves a PHYSICAL bacteria (like FAB I and II) or an ASTRAL organism (like FAB III). While you imply that it is an astral organism, pointing to the symbiotic relationship with FAB III (which I think is a lovely idea), you also say that it can be put into a nutrient suspension and be worn, which is counter-intuitive for an Astral-only entity.

All varieties of FAB are dual natured. FAB 3 is just the only one that can survive without a nutrient source because it creates the nutrients it needs from the mana it consumes.

TO answer some of the questions.
FAB IV functions very similarly to FAB III. FAB IV will provide Spell Defense dice equal to it's Force. These dice refresh as normal per combat turn. Bacteria by nature are very simple organisms and thus not capable of dividing spell defense among multiple magic attackers. It will use all it's spell defense against the first magical attack against it, if you get hit with spells twice in a round, too bad, suck it up.

Once Strain IV has attached itself to a host it begins to feed, since it has originally adapted to a harmless feeding symbiosis with Strain III, when bonded to a metahuman or critter host it's feeding does cause some temproary draining.
The feeding time is based on the Force of the bacteria sample that same as the Strain III Drain Table
Rating -- Base time
1-5 -- 18 hours
6-9 -- 12 hours
10+ -- 6 hours

Make a Force test of the FAB IV colony against TN Magic (for awakened) or Essence (for mundanes and critters) Divide the base time by the successes; after this time the colony drains one point from the affected Attribute and adds it to it's own Force rating. The bacteria will continue to drain it's host until there is no more energy, it is removed, or killed (tough to do). Living victims feel as though mildly fatigued and/or nausious growing worse as strain IV drains energy. If the affected attribute is drained to 0 the host takes Deadly stun and imediately passes out. Magic or Essence lost this way is recovered at a rate of 1 point per hour. A strain IV colony with nothing to feed on starve and loose a 1 Force point per week same as FAB III.
FAB IV can be killed in the same manner as FAB III (of course using spells is that much harder).

As for availability, cost, etc I haven't decided yet. Like psykotisk_overlegen said it's so new that anyone who's researching it is gonna keep it under tight wraps. Perfect run material. Hell, imagine the surprise when corporate ops teams start showing up wearing this stuff in field tests, or if LS or KE started issueing it for magic oriented situations.

FAB 3 is scary because it can subsist on ambient mana and can direct it's own movement to some degree. Once out of the lab, FAB 3 can get almost anywhere. Quality hospitals might be clean enough to keep it out, but you probably won't be free of it until you are in a beta clinic or equivalent. Of course, it's only scary to those with an astral aspect, so some clinics may even be lax in responding to FAB 3 intrusions.

Any character wearing this stuff would likely be slaughtered by mages within minutes. Even if the character is immune to magic, they're still pretty screwed if a shaman decides to "Accident" them while they're setting explosive charges, or simply kill them outright with a physical spell of suitable power. That, or simply pay someone to snipe you.

Of course, an interesting run would be to kill someone with this stuff without letting it out of the vial, and then handing the latter over to a fire elementalist for incineration.

Or if you wanted to kill a mage, create a ring containing Force 12 FAB and the anchoring focus so that it slays mage if removed. Then pass it around as party favors...