微生物學 1861

論大氣中存在的有組織微粒——對自然發生學說的檢驗

路易·巴斯德

他在一隻彎頸燒瓶裡把肉湯煮沸——並證明：瓶中出現的生命是隨塵埃飄進來的，絕非憑空而生。

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In depth · the introduction

兩千年來，人們相信生命能徑直從肉湯與腐物中冒出。一段彎曲的玻璃瓶頸，證明他們錯了。

核心想法

把肉湯擱在外面，幾天之內便會爬滿微小的生命。幾個世紀裡，最順理成章的解釋是：肉湯本身孕育了這些生物——生命從無生命中誕生，即「自然發生」。路易·巴斯德卻認為真相更簡單、也更奇異：空氣裡滿是看不見的活微生物，乘著塵埃飄遊，它們只不過是落了下來、繁殖開來罷了。

要證明這一點，他得讓空氣接觸肉湯、卻把塵埃擋在外面——因為批評者堅稱，密封燒瓶會「敗壞」空氣，於是任何無菌的結果都算作弊。他的答案，是一隻瓶頸被向下彎成 S 形的燒瓶：鵝頸瓶。

它是如何誕生的

巴斯德是化學家，而非生物學家，他是側著身子走進微生物世界的——經由發酵，他證明把葡萄汁變成酒、把牛奶變酸的，是酵母與細菌這些活物，而非單純的化學。這讓他確信空氣攜帶著生命，也把他捲入了那個時代最激烈的生物學爭論。他的主要對手、博物學家費利克斯·普歇是位嚴肅的科學家，自有一套似乎顯示「生命憑空出現」的實驗。

巴斯德的鵝頸瓶是決定性的一擊：煮沸、敞開，卻始終澄澈——直到他把一隻瓶子傾斜，讓肉湯淌進積著塵埃的彎頸，數日之內它便渾濁起來。他還把燒瓶帶上阿爾卑斯山，證明更潔淨、更高處的空氣，播下的染菌要少得多。1862 年，法國科學院為這項工作授予他獎項；就尋常意義而言，此案就此了結。

它為何重要

如果微生物乘空氣而來、並由同類繁衍，那它們就能被擋在外面——單單這一個想法，便重組了醫學與食品。外科醫生學會了不讓微生物進入傷口（李斯特的抗菌術）；醫院學會了滅菌；我們學會了把食物裝罐，把牛奶與葡萄酒「巴氏消毒」——溫和加熱以殺滅微生物，而這個詞，正是巴斯德的名字。而最重要的是，疾病本身從此可以被理解為特定活體生物所致的感染——現代微生物學與大半醫學，皆奠基於此。

一個可以想像的畫面

想想廚房水槽下那段 U 形彎管。它裡頭總存著一點水，正是這個「存水彎」攔住了下水道的臭味與汙濁，不讓它們返回屋裡——而空氣仍可通過。巴斯德的鵝頸是同一巧思的反用：彎處是個陷阱，截住從進來的空氣裡沉降的塵埃與微生物，使它們永遠到不了肉湯。把液體傾進這個陷阱，你就等於把截下的微生物，徑直沖回了自己那份乾淨的肉湯裡。

它的位置

巴斯德站在一場漫長爭論的終點：弗朗切斯科·雷迪早在 1668 年就讓蓋好的肉裡不再生蛆，拉扎羅·斯帕蘭扎尼又在 1765 年煮沸並封存了燒瓶，可兩人都堵不住關於「空氣」的反駁。巴斯德把它堵上了。他的結果，與本館其他文獻直接相通——約翰·斯諾方才論證霍亂經水而傳（1855），約瑟夫·李斯特將把細菌學說帶進外科（1867），而亞歷山大·弗萊明的青黴素（1929），日後將對巴斯德最早在空氣中捕獲的那些微生物，反將一軍。

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Original source text

Louis Pasteur · Mémoire sur les corpuscules organisés qui existent dans l'atmosphère · Annales des sciences naturelles, 4e série, t. 16 · Paris, 1861

Pasteur's thesis: the living things that appear in a boiled, decaying infusion are not generated by the broth itself. They grow from germs — minute organisms suspended in ordinary air, riding on its dust. Keep the dust out, and however freely the air comes and goes, nothing will ever live in the liquid.

The doctrine on trial

For two millennia it was taken for granted that life arises spontaneously from non-living matter — maggots from meat, mice from grain, swarms of microscopic 'animalcules' from any broth left to stand. Redi (1668) had banished the maggots, and Spallanzani (1765) had shown that a sealed, boiled flask stays lifeless; but his critics had a ready answer. Sealing and prolonged boiling, they said, spoils the air or destroys some 'vital force' it carries, so the absence of life proves nothing. To settle it, an experiment was needed that admitted fresh air in full — and excluded only the dust.

The swan-neck flask

Pasteur drew the neck of a flask out in a flame into a long, downward S — a 'swan neck' (col de cygne). He boiled the broth to sterilise it and left the flask open to the air. Air passed freely in and out; but the dust it carried, and the germs upon it, settled in the low bend of the neck and never reached the liquid. The broth stayed clear for months, for years. Then, tilting the flask so the broth ran up into the bend and back, he washed the trapped dust into the liquid — and within days it teemed with life. The air had never been the issue. The dust was.

There is no known circumstance in which it can be affirmed that microscopic beings came into the world without germs, without parents similar to themselves.

[ … ]

Before the Academy

Pasteur reinforced the case from every side. Drawing air through a plug of gun-cotton, he dissolved the plug and found under the microscope a litter of 'organized corpuscles' indistinguishable from the germs that grew in his flasks. Exposing sterile flasks at different places, he found that ordinary cellar and street air seeded almost all of them, while air from a high Alpine glacier seeded very few — the germs were unevenly scattered through the atmosphere, not everywhere and inevitable. For this body of work the Académie des sciences awarded him its prize on spontaneous generation in 1862.

From the 1864 Sorbonne address

Louis Pasteur · « Des générations spontanées », a public lecture · Sorbonne, Paris · 7 April 1864

Never will the doctrine of spontaneous generation recover from the mortal blow that this simple experiment deals it.

Paris · École Normale Supérieure · 1861