I was reading a microbiology textbook and all it said was "The work that began with Pasteur started an explosion of discoveries in microbiology. The period from 1857 to 1914 has been appropriately named the Golden Age of Microbiology." I know that World War 1 began, but it doesn't go into detail to why the Golden Age actually stopped at 1914.
There doesn't need to be any particular event in 1914. The starting and ending dates of historical periods are, to varying extents, generally somewhat arbitrary. It is relatively easy to point at the peak of a movement. At which does its rise constitutes the start of an era, and by which point can we classify its decline as an end, is much more opinion based.
In this case, microbiology experienced a flourish of discoveries in the late 19th century that established it as a modern scientific discipline. It seems that the number of major breakthroughs peaked around the 1880s or so. A brief survey seems to suggest that the pace of discoveries was in a relative decline by the early 20th century. This is not, of course, to say that important research wasn't being carried out, but the rhythm definitely seemed to have settled down. All golden ages have to end and it seems that microbiology's time had come.
However, although it seems apparent that the microbiological golden age had peaked, its decline is ultimately only a trend. There were no major breaks that could be chosen as a naturally obvious end date. That is, until the First World War shattered the Belle Époque. Given this context, the monumental calamity of the Great War seems like quite a logical cut off point for the Golden Age moniker.
See the following tables of of major microbiological discoveries for examples of the aforementioned decline. While this is by no means a comprehensive survey of the discipline, it does illustrate a broad general trend.
(Source: Rao, Dubasi Govardhana. Introduction to Biochemical Engineering. Tata McGraw-Hill Education, 2010.)
A similar pattern of slowing down can be observed in this next list, which dealt exclusively with causative agents of microbiological diseases.
Source: (Kokare, C. R., Pharmaceutical Microbiology Principles and Applications. 6th ed. Pune: Nirali Prakashan; 2008.)
Lastly, two lists on bateriology and virology:
Source: (Nagoba, B. S., Microbiology for Nurses. 2nd ed.)
Not just microbiology, but all sciences virtually went on hold during the war. Research plummeted. Many schools had trouble attracting students, who were being drafted into the war and had to charge less tuition as demand dropped for advanced education. Also, unlike WW2, there were no deferments for students or scientists. If you were a 23-year-old biologist you were just as likely to get conscripted as a 23-year-old welder. Many promising scientists were killed in the trenches. One of the most brilliant physicists of the 20th century, Henry Moseley, was killed when he was 27 on the beaches of Gallipoli. Many other promising young scientists met the same fate.
It was the beginning of World War I, and the global disruption engendered by that war.
It wasn't just the "golden age of microbiology," but almost "the golden age of everything" that ended in 1914. This was succeeded by the "age of catastrophe" that most authorities agree began in 1914, using different ending dates. The two World Wars were hugely destructive of manpower and economies. To a lesser extent, this was true of the depressed 1930s. And even the relatively calm 1920s were a period a "suspicion" and anti-intellectualism characteristic of the interwar period that generally hinder scientific research. And by the 1930s, some of Germany's best (Jewish) scientists were being summarily fired from their posts, and forced to emigrate.
Within "microbiology" there were some exceptions to the rule. One example is virology, which got a relatively late start in the 1890s, and had arguably its most prolific era in the 1920s and 1930s. But that was a "special case," scientifically, that was an exception to the general trend in science and elsewhere.